Last month, I got Thing Explainer. It's a book that explains big things using the ten hundred words that people use the most. I wanted to do something just like that, but for hidden writing. After I started writing this, I felt like I was talking to a little kid, only to find out that I was the kid, and I was having fun.

When I say "hidden", they aren't all that hard to find. It's just that when you find one, you can't tell what it means. There are many ways to hide what you write, but the important ones fall into three groups by how many keys, or numbers, are used to hide messages. The first group uses one key, another group uses two keys, and another group that uses no keys at all.

• One key hiding

This way means that hiding a message and getting it back uses the same key. Most of these ways are very fast, so lots of stored and moving messages use one of these. Sometimes the way in which you hide the message is the same way you get it back, but that's not always the case.

• Two key hiding

Two key hiding means that message hiding uses one key, and getting it back needs a different key. For these to work, you make two keys that go together in some way. One key must be kept to yourself very well, and the other key is given away to anyone that wants it. If the one key is not kept to yourself, all ideas behind two key hiding go away.

One idea allows two people who have never talked before to talk without anyone else knowing what they say. This is important, because the one key way needs both people to know the key used to understand each other. That means that they needed to have talked before. They can use two key hiding to agree on what one key hiding to use.

Another idea is that you can tell where a message has come from. Even though hiding a message and getting it back use different keys, it does not matter which one is used for either. Someone can give you one of their keys, and many years later, give a message to you. You can be sure that it's from them, because if you use their key on it, it will make sense. And if you trust someone's choices, they can promise that another person he knows (but you don't) is who they say they are.

• No key hiding

No key hiding, or one way hiding, is used to make sure a message is the message you think it is. But these are very different from the other two, because you can't get the message back. This might seem crazy on the outside. It's like a dead man: it says nothing. And like a dead man, if you know who someone looks like, and how people can die, you can go after someone for breaking a law.

Have you ever noticed that most computers won't give you your pass words? It's because the computer used no key hiding on them, and turned your words and some of its own into a pretend dead man that only it and you know together. When you want in, the computer turns your and its words into another pretend dead man, and lets you in if it is the old one. This way, if bad people steal everything from a computer, they will have a lot of pretend dead men instead of your words, and should not be able to guess them because some of the computer's words were used to make it, too.

All three ways of hiding messages allow you to read this and many other things on other people's computers, like big number findings, or faces on books, or bird songs.

• Once your computer knows where another computer is, it says 'Hello, I want to talk to you'.

• The other computer replies 'Hello, here's one key used for my two key hiding, and a promise that I am who I say I am'.

• Your computer has some friends it trusts and their keys. It uses no key hiding on a friend's key and the key from the other computer you are trying to talk to. It should be the same as a part of the promise.

If things aren't right, your computer will tell you, and it might turn all red.

If things are OK, it means that your computer trusts the promise that the other computer is what it says it is.

• Your computer picks a one key hiding, and a key to use for it. It hides that with the two key hiding that the other computer told it to, and sends it.

• If the other computer understands and says 'OK', they start talking.

• To make sure that the hidden messages are not changed, they use no key hiding between the one key hidden messages. This way, you (and your computer) are talking to who you think you are talking to, no one can hear what you are talking about, and what you are hearing is what was said.

This does not hide from other people the fact that you are talking, but that is a big and hard problem to fix.

But how do we know that someone isn't able to guess the keys and numbers used? It's because they are so big (much bigger than in the pictures), that if someone were to guess every one of them, they would only guess a small part before they ran out of power and time. It will be this way until hiding machines are built from something other than matter and sit in something other than space. Space doctors say so. Instead, people try to find problems in certain hiding machines used to hide messages to guess, shape, or even steal the numbers used. About 2 years ago, such a problem was found in a hiding machine that about half of all computers use. After this, many computers asked at once for people to make new pass words.

Hiding machines allow almost everyone in the world to use hidden messages. The US had a law that said you can't give hiding machines that used big keys to anyone outside the US, because they said it was like a soldier's special gun. But someone made a big key hiding machine and gave it to anyone anyway. Some law men started going after him, because he broke the law. But computers had changed since that law was written, and it made an interesting problem. Because books count as saying something, and you can say anything you want, he put the directions to build the hiding machine into a book. People around the world could take that book and put the directions into their computer and have a big key hiding machine. But not letting people have a book is very bad, so the law was changed to let anyone have any hiding machine they want.

Hiding messages, especially between every computer, is an important thing these days. It started a few years ago when a special police man gave some news paper people some papers that they should not have. Those papers said that other special police men were reading everything; things that people think they should not be reading. Since he said that hiding messages worked, computer people thought that they could change the world, so they started hiding every message they could. Up until then, hiding messages was hard and often used a lot of money, but computer people are changing this, too. It's good that computers have hidden messages very fast for a long time.

Some police men say that hiding messages is bad, since they fear a time when they can't prove that a bad person is bad, especially when the bad person wants to hurt people or make them feel afraid. Different from what was first said, a large attack a few months ago did not have hidden messages.

But that doesn't happen as often as bad people stealing things like plastic card money from good people. Even though the special police can't read hidden messages, bad people can't read them either.

Hiding messages also helps people against the strong. Strong people (even like law and police people) can know things that others would not like them to know, and strong people can use this to stop others from doing good things. Most law people know this, and think hiding all messages is a good thing.