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Last updated: November 21, 2015

About this site

Who writes this stuff?

All the articles on Explain that Stuff are written by Chris Woodford, a British science writer with over 25 years of experience in explaining science and technology.

Is this site safe for children?

Yes (hopefully). This is an educational website that I hope is safe and suitable for all family users, though it's mainly intended for ages 10+. I take my responsibility to young readers very seriously:

If you discover anything that you think may be inappropriate for young readers, please alert me straight away and I will try to put it right immediately.

External links

Please be aware that virtually all articles on this site contain links to carefully researched external sites for further reading (and they usually open in a new window so you can tell you're leaving this site). When I say "carefully researched external sites," I mean ones I have spent a lot of time looking for, which I hope will build on the information I've provided in my articles—books you might find useful, topical news articles, carefully vetted YouTube videos that demonstrate things right before your eyes, or whatever. I do not mean sneaky attempts to sell you garbage you don't want. There are absolutely no clandestine links of this kind, product placements, or paid links provided by manufacturers anywhere on this site. There are a few places where I link to manufacturers' websites for information about products because I've judged those to be the best sources of information; for balance, I try to include a number of different manufacturers wherever possible. The recommended book links generally take you to Google Books, not Amazon, because in my opinion that site provides the best catalog of the world's books currently available. These book links earn me no money; they're there because they're helpful, not because I am trying to sell you stuff. I go to great lengths only to include links to useful and reputable sites, and I do recheck links periodically (generally, every article is checked at least once a year), but—by the very nature of the Web—I have no control over, or responsibility for the content of, external sites. Again, if you find anything problematic, please do let me know.

Why explain stuff that way?

Here's a word or two about my pedagogical approach, for parents, home-schoolers, teachers and anyone else who's interested.

Our survey says...

Most people think science and technology is fascinating, fun, and incredibly important, yet they don't really understand it, even when they've studied it for years in school. How do I know this? Well... consider the findings of two recent surveys:

So people actually do like science and they do find it interesting, but they don't really understand it and they don't feel properly informed about it: somehow it just flies straight over their heads. That may be why so many children convince themselves science is hateful or boring in school... only to discover years later, as adults, that they actually quite like it! Whose fault is all this? I'm not sure—but my mission is to help put things right.

What I try to do is present often quite complex stuff in a way ordinary people can get a handle on. You'll find my articles are a bit different from the ones you get on other websites or in science books: I try to give you a good, clear, simple understanding of a subject rather than drown you in facts, details, and nit-picky trivia. Generally, my approach is to talk you gently around a subject, building on familiar stuff you're likely to understand already, a bit like a decent teacher would do.

Why simplest is best

After studying science for about 35 years, and writing about it for over 25, I firmly believe that it's better to completely understand a small amount about something—the essence of how something works or the science behind it—than to know vast amounts of non-essential details that don't fit together in your mind and don't make sense. In my book (and some people will strongly disagree with this), it's even okay to have a slightly wrong or oversimplified understanding of something than a totally correct misunderstanding. So, for example, I think it's much better if children have a firm, oversimplified understanding that electrons "whiz around" atoms that they can explain to a friend than some sort of fuzzy, confused (but scientifically more accurate) idea about orbitals and probability that they barely understand and could never explain to anyone else. This is not an excuse for dumbing down: it's an argument for teaching with sensitivity—understanding your audience and what they need to know. You can always build on a firm, simplified understanding (or correct, tweak, or qualify it) later on; you can never build on confusion. And if you turn people off too soon, you may well lose them forever. What good is that?

Learning lessons from other websites

This website is not supposed to be anything remotely like Wikipedia—and for a good reason. I value Wikipedia very highly and I think it's an absolutely superb contribution to collective knowledge. But it has serious shortcomings as an educational tool because it fails to take account of a simple fact that is blindingly obvious to any teacher: readers of different ages and abilities have very different levels of understanding and need very different kinds of information. Many Wikipedia articles (especially the scientific and technical ones) are now written by self-professed "experts," for themselves and their friends—people who take too little time to consider how to explain things clearly to general readers (or young students). As time rolls on, simple outline articles ("stubs") become longer and more complete, but they often become more complex and convoluted and harder to understand at the same time. (One of my favorite examples is Wikipedia: Entropy, an article so impenetrable to general readers that it has now spawned an offspring called Wikipedia: Introduction to Entropy, which, in my opinion, now merits its own "Wikipedia: Introduction to Introduction to Entropy" that ordinary people (and students under the age of 18) might just have a hope of understanding. I say this as someone who studied thermodynamics to degree level. That's just one random example. Take a look at Wikipedia: Refractive Index for another example. Lots of great information, if you happen to have a degree in physics, but most of it entirely unsuitable for most of the readers who will look at it—typically, I would guess, younger teenagers seeking homework help.) In short, Wikipedia's problem is that its writers start off from what they're burning to say rather than from what people want and need to know; a good teacher or educator will always start from the audience.

Example of Wikipedia's 'too technical' labeling

Artwork: Wikipedia is waking up to the fact that many of its articles are too complex for general readers. There's now a handy template you can add to flag up the problem for a fix: simply edit the problem section and insert {{technical}} to make this little box pop up in the text. Add a note on the "talk" page, if you wish, explaining why you feel the material you've flagged is "too technical" to understand.

Of course there are plenty of websites besides Wikipedia, but quite a lot of them merely describe gadgets and technologies without explaining them or putting them into a broader context. That's sometimes because the people who write them don't have a rich enough scientific understanding. But it's also because many websites contain vast numbers of bitty articles written by separate authors and, unlike in book publishing, there's no editor to give the whole thing an overall structure and ensure all the articles work together as a coherent whole. My approach is much more book-like. The emphasis here is on gently building knowledge and understanding, not filling people's heads with undigested confusion. Apart from explaining how things work, I try to get across the fundamental scientific principles behind things so you can make the connections, see the patterns, and figure out how everything links together. So while I have (for example) articles about toasters, loudspeakers, and refrigerators, they're also quietly teaching you the science of heat, electromagnetism, and thermodynamics. Learn the basic science behind a few things and you can figure out the technology of quite a lot of other things. This website is book-like in another important way too: I write everything on the site, so I know exactly how all the articles work together. Over a number of years, I've figured out which articles I need to add to make a very comprehensive collection that works as a coherent whole, in much the same way as a printed book. In one important way, the website is very unlike a book: it's being continually revised and improved to take account of new information and feedback from readers. Every article clearly states at the start when it was last updated.

Learning from history

Tech websites have a very unfortunate obsession with newness and fads; Twitter (for all its good points) encourages a tendency to recycle ("share") items of current science news, often making matters worse. How many people who fell over themselves to "retweet" stories about neutrinos breaking the speed of light had the slightest idea why the probability of that scientific finding being correct was so incredibly slim? In sciences such as physics, chemistry, and biology, the core, bedrock stuff you need to learn is years, decades, or even hundreds of years old and much of it will never change. (That's a measure of how excellent the work done by people like Newton, Maxwell, and Einstein really was.) In a subject like physics, the stuff people learned in schools and colleges 50 years ago is still the essence of what children need to understand properly today before they surf the flotsam and jetsam of the new. That's why we can (and should) still read and admire lectures delivered in the 1960s by such great scientist-educators as Richard Feynman; they are just as relevant today. You can't properly grasp any invention, machine, or cutting-edge technology if you don't properly understand concepts like electricity, magnetism, heat, atomic structure, the laws of motion, the kinetic theory, the conservation of energy... and so on. Those are the sort of topics that I drum home on this website, carefully disguising them as articles about interesting everyday things!

Even for cutting-edge science and technology, historical background is really important. That's why there's a full timeline on this site, indexing most of the inventions and discoveries chronologically. Over the last couple of years, I've also come to the conclusion that inventions can be explained more richly by referring to original patent drawings, particularly if they're color-coded and annotated simply. If you want to understand an invention like the laser printer, you can do it by looking at the components inside and seeing how they form a logical sequence. But you can also do it by going back to Gary Starkweather's (the inventor's) original patent and noticing that he invented the laser printer by bolting a laser scanner on top of a conventional photocopier. That explains why laser printers and photocopiers have so much in common. A lot of my articles now contain details of patents to give readers a richer understanding of how things worked and why inventors solved problems a particular way.

Anyway, that's where I'm coming from—and it's quite enough pedagogy for now! In a nutshell: this site is dedicated to helping people learn useful things about everyday stuff in an interesting way. I've spent over six years (so far!) putting it together and I really do hope you enjoy it. If not, do please write and tell me. Good or bad, your feedback is always interesting to me and always welcome.

How does this website support curriculum learning and home-schooling?

You'll find detailed help in my teaching guide, which shows how my articles map to different parts of a typical science curriculum for students aged 10–18.

Why do you only write about science and technology?

I want the material on the website to be of a high standard, so I write about only the things I feel reasonably well qualified or knowledgeable enough to write about: physical sciences, technology, engineering, environmental issues, and computing. To write a really good explanation, you have to understand something very well indeed; when people write about things they don't understand, it's very obvious—and it's not fair on readers. Unfortunately, I don't know enough about life sciences, economics, law, politics, and so on to write well about those other topics. Maybe at some time in the future I will come across someone who can do those things for me!

Why do you spell things the American way?

Although this site has a worldwide audience, more readers come from North America (the United States and Canada) than from anywhere else. For that reason, I do my best to use US editorial conventions (spellings, punctuation, and so on) wherever possible and as consistently as I can. From a publishing viewpoint, that means Webster's spellings and, wherever possible, Chicago grammatical conventions. Occasionally I receive complaints from people who object to words like "aluminum" (on the grounds that that's not the official IUPAC convention) or "meter" (see below) but I'm afraid that's simply the convention I've adopted to satisfy a majority of my readers.

Why do you use metric units? Why do you spell SI units "meter" and "liter"?

This is a scientific website so I follow the convention of using metric and SI units (such as meters and kilograms), though I generally give Imperial conversions in brackets. Like many US science publications and textbooks, I have decided to follow the US National Institute of Standards and Technology convention, based on Webster's (US English), rather than the Oxford (UK English) convention:

"(i) The spelling of English words is in accordance with the United States Government Printing Office Style Manual, which follows Webster's Third New International Dictionary rather than the Oxford Dictionary. Thus the spellings "meter," "liter," and "deka" are used rather than "metre," "litre," and "deca" as in the original BIPM English text".

Using material from this website

Please note that different rights apply to the words and the pictures on this website. Briefly:

Here's a more detailed explanation if you need it...

Using words from this site

Can you copy and use articles from this website on your own website? No. I'm very sorry, but I do not permit people to copy articles from onto other websites (including blogs, Facebook pages, and other personal sites) for any reason whatsoever (including translation into other languages).

Fair use ("fair dealing"), such as briefly quoting from my articles for purposes of commentary or discussion, is fine, providing you cite this website as the source of your quote. Fair use does not allow you to republish extensive or entire copies of our articles and use them as you wish on other websites, or elsewhere, even if you're doing so for noncommercial or educational purposes. Please note that I will generally regard copying anything more than about a couple of paragraphs and any unattributed quotation (passing my material off as your own) as copyright infringement. Please be aware that all the articles on this website are either registered at the US Copyright Office (or deposited there pending registration), which means deliberate copyright infringement can make you liable for severe financial and even criminal penalties. I routinely scan for unlawful copies of my articles and I take some kind of action in every case of copyright infringement I find. I take this matter extremely seriously and employ law firms in Europe and North America to help me deal with the problem.

The simplest way to avoid any kind of copyright problem or misunderstanding is to ask for permission. Please feel entirely free to contact me if you have any queries, you'd like to discuss reprinting or reusing our articles, or you'd like me to confirm whether some use you have in mind is "fair use" or otherwise okay. I am always happy to help—and generally I'll reply within a day or so.

Using pictures from this site

Can you copy and use our pictures? Broadly speaking, if you're not using them to make money, yes, with pleasure! But do read on.

The photos and artworks on this site fall into three different groups:

  1. Any images and photos used on this website are believed, in good faith, to be in the public domain if they are credited to US Federal Government agencies such as NASA, the US Department of Energy, or the US Military (US Army, US Air Force, US Navy, US Marines), or their employees. I have taken reasonable steps to check each image, but I cannot and do not guarantee its copyright status. If you want to reuse one of these photos, it's your responsibility to confirm the copyright status for yourself. In case you're wondering, I have sometimes "cropped," blurred, scrambled, or otherwise disguised people's faces in public domain photos to protect their right to privacy. It is my policy to give individual photographers full credit for their work whenever I possibly can and I would suggest you do the same.
  2. I also use a few photos from Flickr that are published under various Creative Commons licenses. Please note that these photos are still protected by copyright and their owners kindly grant us permission to use them, but only if we follow their conditions (the licence, in other words). If you wish to reuse these photos on your own website, please be sure to follow the licence exactly. Usually that means you have to credit the original photographer (which is only fair) and repeat the licence. If you're not sure, copy the exact words I've used to credit the photographer and the link to the Flickr and Creative Commons websites and you should be OK. (I return the favor to the Flickr community by offering some of my own photos for reuse on our Flickr page, all under a Creative Commons licence.)
  3. Creative Commons License Any other photos or artworks that are not attributed are ones I have created and you're welcome to use and reuse them for noncommercial purposes only under the terms of the Creative Commons NC-SA license. For the avoidance of doubt, please note that "noncommercial" means you may not charge a fee for the use of my work, use it in any for-profit publication (whether educational or not), use it to sell, endorse, or promote any product or service, or earn any money from it by advertising or any other means whatosever, irrespective of whether you make a profit. Hi-res versions of a few of my photos are available for commercial use, under license, on our Flickr page; other photos are available in high resolution on request. Please would you kindly attribute any images I've created by crediting and, preferably, making a link to the page on my website where the original image appears (a "nofollowed" link is perfectly fine). Some HTML along these lines would be just great, thanks:

    Image from <a href="">Explain that Stuff</A> published under a <a href="">Creative Commons License</a>.

Hotlinking to pictures from your pages

Hotlink artwork

If you would like to use images from this site, you'll need to copy and upload them to your own server rather than linking directly to them ("hotlinking"). Please note that image hotlinks to are automatically blocked. If you link to my images directly from your own pages, you'll find a dummy placeholder image (right) appearing instead of the picture you actually want. Or you may find your images suddenly disappear without warning when I change the image server address (as I do from time to time). Worry not. To get the correct image, all you need to do is copy it to your own webspace: right click on the image you want, save it to your hard disk, and then upload it to your own web server as though it were a file you created yourself. If you have problems, send me an email and I'll try to help.

Would you like to license photos or artworks for commercial use?

Privacy and cookies policy

You'll find the full privacy and cookies policy for this website on the privacy policy page.

Permission to reprint in books

Since this website is all about science education, I'm happy to discuss requests for limited reuse of material from this website in educational books and other materials, but (as explained above) not on other websites or in any other material that appears online. Now all books are educational in some sense, but by "educational books" I mean very specifically textbooks, other books geared to school and college students, or those primarily produced by the author for educational reasons.

Please kindly send me details of your publication, intended audience, approximate print run, publisher, and anything else you think might be relevant, and specific details of the material you'd like to use, and I'll let you know what's possible (usually the same day). Please note that if your use is commercial (for profit), I will normally charge a licensing fee. If you're an educational publisher producing low-volume books only for schools, the licensing fee is normally reduced by 50 percent. If you're a nonprofit or charity educational publisher, I don't normally charge a licensing fee, but you will still need to request permission. Please see my licensing page for more details about licensing fees and commercial use.

How (and when) should I cite your articles?

When to cite

It's often a good idea to acknowledge the source of articles you use—and absolutely required by many schools, colleges, and other academic instutitions. If you're quoting verbatim from something, citing a source is the way to avoid problems of plagiarism; it's also generally a requirement if you're claiming "fair use" (fair dealing) of copyright material. The way to cite a web page varies according to the citation style you're using (if no-one has told you which style to use, it doesn't matter). Most of our articles have ready made cut-and-paste citations at the bottom of the page. A few articles may still be missing them. In that case, please use a citation like this in your bibliography, references, or footnotes:


For example:

When not to cite

Wikipedia users have become obsessed with finding sources and citations, which is generally a good thing—but only if you understand the difference between reliable and unreliable sources. I do not claim to be an expert on many things; I am an educator: my articles are general and educational and should not be cited as definitive sources of factual information (Beethoven's birthday, the population of Fiji, the melting point of iron).

I am a teacher. Can I print and use material from the site in class handouts?

Yes. If you are teacher or professional educator, you are most welcome to print and circulate up to 100 copies of any article as a handout (or other teaching material) for classes within your own school or college, providing you do so for free. You do not need to ask permission, but please kindly credit this website as your source. I consider this a perfectly fair use—and something I would warmly endorse and encourage. However, please kindly do not copy articles verbatim onto publicly accessible school or educational websites.

Can I use material from this site inside my company (purely for an internal audience)?

Subject to the restrictions noted here, yes. If you're producing educational material for use purely inside your own company or corporate group (not for any kind of general external or customer publication), you may print and circulate up to 100 copies of any one article from this site as a handout (or related teaching material), providing you do so for free. You do not need to ask permission, but please kindly credit this website as your source. However, if you'd like to reproduce any material in any publicly accessible form (such as customer literature), you must seek permission first and (usually) enter into a licensing agreement, for which you will normally be charged a licensing fee. Please kindly note that I do not give permission for the reuse of material from this site on company websites for any reason whatsoever and any unauthorized use will be regarded as copyright infringement (fair use excepted, such as briefly quoting from my articles for purposes of commentary or discussion).

Can I translate your articles into another language and republish them on my website?

Sorry, no. I do not permit reuse of articles from this site on other websites, either in English or in any other language.

Copyright and legal information

You'll find this on the copyright and legal notice page.

Why is your website running intrusive advertisements that seize control of my computer?

As far as I know, it isn't! This website runs as little advertising as it possibly can (two relatively small ad squares) to help pay its costs. It runs only conventional advertisements and nothing more intrusive than Flash. Occasionally, people contact me to say that they have seen very intrusive or distracting advertising while viewing this site. does not run intrusive advertising of this kind. There are no pop-up or pop-under ads, no ads that seize control of your browser, and no exit-type or "sneaky redirect" ads that redirect you to another site. If you're seeing ads like this, it means your browser or computer has been hijacked by malware (malicious software). You need to run antivirus and anti-malware scanners immediately (such as Microsoft Security Essentials and the Windows Malicious Software Removal Tool). If you're running Windows, please check out Microsoft's guide to fixing a hijacked web browser.

I've been annoyed or offended by an advertisement on your site

Compared to similar websites, this site runs an absolute minimum amount of advertising to help pay the cost of developing it. Advertisements are currently run for Explainthatstuff by Google's Adsense system (several million ads each month). I have no direct control over exactly which ads run, or when they do so, though I have blocked a number of broad categories of advertising that I consider inappropriate for an educational site. I can block irritating, annoying, or unsuitable ads, but, unfortunately, only after they have started running and only when people make me aware of them. If you see something that bothers you, please accept my very sincere apologies. Please kindly send me details (better still, a screenshot of the ad if you can possibly manage that) and I will try to block it from running on the site at the earliest opportunity.

Advertising on this website

Sorry, you cannot place direct advertisements on this website. does not accept any direct advertising at the moment; all the advertising is coordinated through Google Adsense. does not sell links: you cannot pay to place text links anywhere on this site (even "nofollowed" ones). I do not, never have, and never will sell links and I will not reply to any emails I receive asking to buy links.

Would you like to switch your advertising to our network?

No. Thank you for considering this site, but I am very definitely not interested in using any other advertising providers at this time and I will not run trials of other ad networks. I'm very sorry, but I will not reply to speculative emails from advertising providers.

Would Explain that Stuff like to join my affiliate scheme?

No. I am not interested in affiliate schemes, referrals, or any other remotely similar programs.

Link policy

The answer to almost every question I receive about links is no.

I'd like to buy this website/domain. Is it for sale?


Will you syndicate your articles to us to publish on our site?

No. I'm very sorry, but I am not interested in syndicating articles to other websites. I do not allow my material to be copied or published by other websites for any reason (other than legitimate "fair use") and I will take action against unlawfully copied material whenever I find it.

Will you license your articles to us to use offline?

Maybe. Please see the licensing page for more details.

How do I license a photo/artwork for commercial use?

Please see the licensing page for more details.

Do you accept articles or submissions written by other people?

No. Sorry, but I only publish my own material. There are absolutely no exceptions to this. Thanks for the thought, but I am not interested in publishing articles by anyone else, for any reason, even if they're supplied for "free."

Will you write something for me?

Maybe, though I have very little spare time. If you're a nonprofit organization and you're doing work on science or environmental education, please do get in touch. If you're a company and you'd like me to write some sort of promotional, commercial, or technical material for you, it's unlikely that I will be able to help; although I used to work as a copywriter, I no longer really do marketing and promotional work.

Would you please remove me from your email list?

As it says very clearly in the privacy policy, Explain that Stuff doesn't collect information about you and there is no "email list" from which you can be removed. If you receive a spam email claiming to originate from, it's a fake; I do not send bulk/spam emails. Similarly, if you receive any kind of "phishing" email from asking you for personal information, that's a fake too; I don't send any emails of this kind. Check the full headers on the email and you'll find the originating domain has been forged. Unfortunately, as banks and other organizations around the world have found, there's little or nothing to stop people from sending forged emails—and certainly nothing I can do about it; please simply delete the mail... and move on.

Contacting us

Before emailing, please quickly check that your question hasn't already been answered in the FAQ up above. Thanks!

I'm very sorry, but I cannot provide extra information about any of the articles on this website or help students with projects, essays, or schoolwork. I'm really sorry about that, but there just aren't enough hours in the day and school assignments are meant to test your abilities, not mine. By writing and publishing articles on this website, I can attempt to help lots of people; if I spend all day long replying to emails, I don't use my time anything like as productively.

Found a mistake?

If you find any errors or inacccuracies in any of the articles, if something really confuses you and you think I should reword it, or if you'd like to suggest any other improvements, please do get in touch. I'd be very pleased and grateful to hear from you. I do my best to fix mistakes immediately (usually the same day I hear about them).

You can send me a general email using the email form; to send feedback on an article, please use the feedback form.

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