A computer will do exactly what you told it. Not what you meant, not what would obviously be sensible — exactly what you told it, billions of times a second, without ever once wondering whether that was a good idea.

In 1999 a spacecraft flew nine months to Mars and burned up on arrival. Nothing was broken. Two teams of excellent engineers had each written correct software, and the two programs disagreed about one thing neither of them ever said out loud.

Every photo, every song, every message you have ever sent is stored as nothing but switches that are on or off. There is no third option anywhere in the machine — and by the end of this you will be able to count to 31 on one hand.

Search a billion things and the answer comes back before you have let go of the key. The obvious explanation is that the computer looked at a billion things very quickly. It did not. It looked at about thirty.

A well-built website does not know your password. It has never known it. It cannot tell you what it is, cannot look it up, and cannot be persuaded — and it still recognises you every single time you type it.

Ask experienced programmers what the hardest thing in their job is and a startling number of them say the same thing: dates. Not graphics, not security, not speed. Working out what day it is.

Get a grown-up, a piece of paper and a pen. You are going to tell them how to draw a cat, they are going to do exactly what you say, and the cat is going to be terrible. That is the point.