“Suppose you are reading a book about computers, and you want to find every place in the book where it talks about a man called Mr Turing. (Mr Turing knew a whole lot about numbers. He lived before we had computers, but some of the things that he said about numbers are very important if you want to understand how computers work. So if you were reading a computer book and you would want to read about him.) So how do you find all the places in the book where Mr Turing is?

Well, you can go through the whole book looking at each word and seeing if it is ‘Turing’. But that would take a very long time. You would have to read the whole book. So the people who make books have already done this for you, and if you go to the back of the book you will find a whole set of the important words that are in the book, all in the right order, so you can find them easily And next to each word it tells you exactly where in the book to look. So you would go to the back of the book, and find the place where Mr Turing’s name was. And next to that it would tell you all of the places in the rest of the book that talked about Mr Turing, so you could go straight to those places without having to look at the part of the book that you didn’t want to read.

Now you may think that to build this set of words at the back of the book would take a long time, and you would be right. Someone had to read through the whole book, and mark all the places with Mr Turing’s name. And they would have to mark all of the places for all of the other computer words that people might want to find out about , like “number” or “bus” or “punch card”. This would take a lot of time do do. But once that person has finished making the set of words then everyone else who had read the book would be able to use it.

This is a very important point. Suppose that it takes 10 days for one person to read through the book and find all the important words and note down where they all are. Ten days is a long time. But if you, or I, want to find where Mr Turning is, we can use the set of words to find the right place, and we can do it in less than a minute. If we didn’t have the set of words, it might take us a day to read through the book. And a book might have ten to the power of four people reading it, so if each person had to read through it to find where Mr Turing was that would be ten to the power of four days in total. So you can see that is is much better to have one person spend ten days making this set instead of ten to the power of four people looking through the book.

I work for a team that does the same thing with computers. There are lots of computers on line and these computers have lots of words on them. If you took all the words in all these computers it would be more words than in ten to the power of seven books. There are also pictures as well, and many of these are pictures of cats, or of people without their clothes on, because for some reason other people like to look at these. But there are also lots of important words and pictures out there, such as words and pictures about Mr Turing. Suppose you wanted to find all of the computers out there that had something to say about Mr Turing?

Well, you could go to each computer, and ask it for all of its words and pictures, and look in those words for Mr Turing. But that would take years (yes, really), and you would have to look at a lot of cat pictures. So what you really need is something like the set of words at the back of the book, which would tell you which computer knew something about Mr Turing. Then you could go and ask just these computers, rather than having to ask every computer in the world.

That is what I do. I work for a big team that has a lot of computers. These computers talk to every computer on the world, and find out all the words they know. Then, when you want to find out about Mr Turing, you ask our computers first, and they will tell you what computer you actually need to talk to.

Talking to all of the computers in the world is still a lot of work. But it is much faster if we do it once and then let lots and lots of other people come and ask us where everything is.

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