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The FREE online science and technology book

CCD chip from a webcam.

Model of a steam turbine at Think Tank science museum, Birmingham, England.

Want to know why giant ships can float, how your earbuds make music, what graphene is, or how windows can clean themselves? You've come to the right place! Here you'll find simple explanations you can really understand—hurrah!

Hard stuff... made simple!

Explain that Stuff is an online book written by science writer Chris Woodford (author of many popular science books for adults and children). It includes over 400 easy-to-understand articles, richly illustrated with over 4000 photos, artworks, and animations, covering how things work, cutting-edge science, cool gadgets, and computers. We take the "pain" from explain and the "tough" out of stuff! There's more information on this website than in your average expensive science book, it's continually updated, and it's completely free to use! Explain that Stuff also helps to support curriculum learning (conventional STEM education and home-schooling).

How to use this site

There are five simple ways to find what you want:

Icon for browsing A-Z.  A-Z index: Browse articles by name, from accelerometers to zeolites... via induction motors, LCDs, and regenerative brakes.

Icon for browsing by date  Timeline: Find inventions by date, from the prehistoric birth of solar energy to the development of quantum computers.

Icon for random article  Random: Discover something new. Keep hitting "random" till you find something you like.

Icon for search engine  Search: Use our safe, Google-powered search engine.

Icon for loving STEM  Teaching guide: Browse a typical school curriculum to match articles from this site to topics you want to study or teach.

What's hot in May 2023?

In the news now...

Conceptual artwork of a warming world.

Climate change

In a hint that climate change is accelerating, the latest scientific projections suggest the world could temporarily hit 1.5°C of global warming by 2027. What is climate change and why should it worry us? [News story].

The base (first, S-IC stage) of a Saturn V rocket showing five red engines.

Space rockets

SpaceX launched Starship, the most powerful rocket ever built, on April 20, but it exploded shortly after launch. How do rockets work and why do they explode? Find out in our ever-popular briefing! [News story].

Typical microcontroller microchip in a dual-in-line package.

Integrated circuits

RIP Gordon Moore, who gave his name to "Moore's Law"—the idea that computer chips roughly double in power every year or two (because the number of components packed into them also doubles). [News story].

Concept of carbon capture and storage.

Carbon capture

Tighter emissions controls for dirty, fossil-fueled power plants are prompting new interest in carbon capture and storage (catching carbon dioxide emissions before they escape into the air). [News story]

Concept of artificial intelligence: an AI dummy superimposed on a green circuit board.

Artificial intelligence

Microsoft's Bing search engine has leaped ahead of its rival, Google, with help from ChatGPT, the AI chatbot, which can answer more complex questions. What is artificial intelligence... and why does it matter? [News story]

Water droplets on a lily of the valley plant


Scientists from University College London have just created a new type of glassy, "amorphous" ice, with a random structure and a density closer to that of water. If you're not sure what makes water so special, check out our ever-popular introduction. [News story]

What's new?

We added a few brand new articles in 2022... and there are more to come in 2023:

Concept of artificial intelligence: an AI dummy superimposed on a green circuit board.

Artificial intelligence

Can we imagine a world where computers are smart enough not to need us? Is "human intelligence" a good starting point for designing useful machines?

The force of gravity is slightly greater at the equator than at the poles airplane.


It's taken over two millenia for people to understand how gravity holds the universe together. What do we know so far about this most mysterious of forces?

Orange Schlieren photograph of two supersonic jets flying upward

History of flight

Planes can trace their history back thousands of years to ancient myths and legends. Why have humans always dreamed of soaring to the sky... and why did it take us so long?

Giant pulley wheels in a large crane


The science behind cranes is easy to understand, but why are there so many different types? How much can they lift... and what stops them toppling over?

Aristotle pictured at the National Academy of Sciences, Washington, D.C.

History of electricity

Electricity is an ancient science that powers our modern world. Why did it take so long to figure out how it works and put it to practical use? Find out in our sparky story of electric power!

Black laptop computer loading software from an open red DVD


Computers are wonderful machines you can reprogram to do almost anything. Learn more about algorithms, computer languages, and the basic principles of coding in our simple guide.

Silverback gorilla eating bamboo.

Great psychology experiments

It's no big surprise that other people aren't quite what they seem. But the astonishing finding from modern psychology is that even our own minds are strange and complex things we don't fully understand.

Experiment exposing organisms to blue excitation lighting

What is science—and why does it matter?

From atoms and cells to nuclear power plants and the human brain, the scope of science is truly amazing. But what exactly is the scientific method... and why is it still our best hope for making sense of the world?

Gray plastic Hitachi transistor radio

History of communication

How did we get from the alphabet to the Internet in about three to four thousand years? Follow the story of shared information from cuneiform and hieroglyphs, through radio and TV, to modern-day streaming media and the World Wide Web.

Most popular

These are some more of our classic, ever-popular articles:

Photo of red tractor spraying crops with pesticide or insecticide.

Water pollution

Rivers and seas take a long time to recover from the effects of careless human treatment. What causes pollution and what can we do to stop it?

White electricity plug with two pins.


The most versatile and useful form of energy in our world, electricity is going to become a whole lot more important in future.

The tips of the atomic force microscope (AFM) probe used on the NASA Mars Phoenix Lander.


Can we build a brave new world just by shuffling atoms and molecules under a microscope?

Horseshoe magnet


One of the first bits of science people studied, magnetism is still just as relevant today in everything from electric cars to body scans at the hospital.

Rack and pinion gear made with an erector set


Wheels with teeth carved around them can make you go faster or bump up your power—and here's how.

A dry cell Ever Ready battery.


We all need electricity, wherever we happen to be, so thank goodness for batteries—miniature power plants you can carry in your pocket.

A large electric motor from an electric lawn mower

Electric motors

These amazing machines turn electricity and magnetism into movement, powering everything from handheld toothbrushes to bikes, cars, and trains.

Conceptual artwork of a warming world.

Global warming

Climate scientists recently issued their strongest ever warning about the growing risks of irreversible "global warming." If you find the subject baffling, try our easy-to-follow introduction.

Composite photo showing NASA supercomputer floating in a cloud

Cloud computing

Why buy yourself an expensive computer or programs to go with it when you can get access to something just as good over the Internet? What are the benefits and drawbacks of working in "the cloud"?

What else is on our site?

The articles on our site are divided up into broad topical areas, listed below. We've also given you a rough idea of the kind of questions you're going to find answers to in each section:

Basic digital camera


Why do we bounce telephone calls off satellites? How do cellphones work? What's the difference between digital radio and ordinary radio?

Typical microcontroller microchip in a dual-in-line package.


How can you make a computer think like a human brain? Who invented the computer—and why on Earth did it take them so long?

A digital voltmeter standing with red and black probes attached, wired into a circuit to test a simple 1.5-volt battery.

Electricity and electronics

What's the difference between "electric" and "electronic"? How can you make coffee with a stream of electrons? How can magnets detect burglars in your home?

Blue flame of a natural gas cooker.


How does wind energy come from the Sun? Is nuclear power safe or not? Will the world ever run out of energy? Why are we still so dependent on "fossil fuels"?

Small steel spring flexed in a curve between two fingers.


What stops a bridge falling over? Why can a person lift more stuff with a crane than with their bare hands? What's the difference between hydraulics and pneumatics and why would you use one instead of the other?

Organic vegetable box


Can we still stop dangerous climate change? What causes air and water pollution and how can the world clean up its act? Is organic food really "better"—and, if so, why?

Sony Bravia flatscreen TV


How does a flash memory card store your holiday snaps? Why can a synthesizer mimic any musical instrument that's ever been invented (and even ones that haven't been)?

Blue, self-cleaning, heat-reflecting glass keeps a conservatory cool.

Home life

How can you make windows turn dark at the flick of a switch? If an energy-saving lamp saves energy, how come it still makes exactly the same amount of light?

Blue, self-cleaning, heat-reflecting glass keeps a conservatory cool.


If you had a piece of platinum as big as a man, how much would it be worth? Why doesn't a dry-stone wall fall over? Is it true that glass is a kind of liquid?

A rainbow-spectrum of colors on the surface of a soap bubble produced by thin-film interference.


Which part of a candle flame is the hottest? Why does chocolate actually taste good? Where's the best place to sit on a rollercoaster if you want to scare yourself to death?

Close-up of hydraulic rams on a digger

Tools, instruments, and measurement

How does an iris scanner recognize your eyes? Why don't fireproof clothes catch fire? Can a robot really learn to play drums?

Front view of huge USAF C-17 globemaster plane waiting on a runway


If a man can't fly, how can a plane fly if it weighs as much as 5000 men? How come we say a wheel reduces friction if it's got a tire that grips the road? Why do huge ships float when even tiny bits of metal sink?

Who uses this site?

Optical mouse circuit board with red LED light shining through light guide.

In the ~17 years we've been online (as of February 2023), pages from this site have been viewed over 140 million times in total. Our articles have been cited by dozens of books and over 2000 academic papers and patents. We're one of the world's top 10,000 most-cited websites, according to the Majestic Million. We've received over 4000 user feedback forms in the last decade and 92% gave us four or five stars out of five—a weighted average of 4.6—but there's always plenty of room to do better. We hope you find this site useful too!

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Please do NOT copy our articles onto blogs and other websites

Articles from this website are registered at the US Copyright Office. Copying or otherwise using registered works without permission, removing this or other copyright notices, and/or infringing related rights could make you liable to severe civil or criminal penalties.

Text on this website is copyright © Chris Woodford with copyright dates as noted in each individual article. All rights reserved. Full copyright notice and terms of use.

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