Technology

Cold is not something a machine can make. A fridge can only move heat somewhere else, which is why the back of one is warm.

Turn a computer off and on again and your files are still there. Something has to remember them while the power is gone, and there are three ways.

A clock does not measure time. It counts something that repeats, and the whole difficulty is finding something steady enough to be worth counting.

A mouse measures the difference between two pictures of your desk. Nothing tells it what that difference is — it has to go looking for it.

A mouse has no idea where it is. All it can work out is how far it has moved since a moment ago, and that turns out to be enough.

Your screen makes a picture out of light. Paper makes none, so a printer has to put a physical substance on the page without smudging it.

People talk about computer memory as one thing. It is two, and they work in opposite ways. Slow and full are completely different problems.

A key is not a shape that fits a hole. It is a row of heights, and each one lifts a hidden pin by exactly the right amount.

The internet is a giant network of connected computers. It keeps working because there is rarely just one path between any two of them.

Everyone says the spinning wheels hold it up. Take the spin away and a bicycle still stays up, so that cannot be the answer.

A hard drive remembers with magnetism. Flash memory remembers by shutting a little electricity in a room with no doors and walking away.

Electricity will not travel along a wire that stops in mid-air. Close the loop and the bulb lights. Leave one gap and nothing happens at all.

Earth behaves like a giant magnet. Rub a needle on a magnet, float it on water, and it turns to line up with that field and point North.

Simple machines do not give you extra strength. They change the way you have to push, so the job takes less effort out of you.

Heat wrecks a magnet, and Earth's core is thousands of degrees past the wrecking point. So the thing turning your needle cannot be a magnet at all.

Nothing is free: push with a quarter of the force and your hand travels four times as far. Every simple machine trades one for the other and never cheats.

A computer understands only numbers. Every coding language exists so that people can write something people can read.

Every language exists so a machine can turn writing into numbers. Here is the turning itself, on one line of code, one instruction at a time.

Video games are older than home computers and older than the internet. The first ones were scientists messing about with laboratory equipment.

Make a ball bounce on a web page using CSS keyframes — then change one number and watch the bounce change with it.

Write a calculator in Python that adds, subtracts, multiplies and divides. It is about twenty lines, and you can explain every one of them.

A website is a file on your computer. That is the whole secret. Write it in HTML, save it, open it in a browser. Nothing to install.

Write a pet that gets hungry and tired. The first two programs run right here on this page, so you can predict the answer and then check it.

In Scratch you drag colored blocks instead of typing, so you cannot misspell. The only thing left to get wrong is the thinking.