For the purposes of my second year mathematics essay I've been reading about coding theory. This is the field that I'd like to be in when I'm all grown up, but I stumbled upon something really cool:
Grab the book closest to you, it doesn't matter what it is just so long as it has an ISBN too (you may also want to grab a calculator for the next part).
Right, take the first digit of the ISBN multiply it by ten and add it to the second multiplied by nine, add it to the third multiplied by eight and so on all the way to the last digit (if the last is an X add 10). Then divide this number by 11, as long as you've done all the computation correctly then you will always get an integer! How cool is that, error detection in ISBNs!
If you want to know how that works, either do a Google search, or wait a week or two until I've written my essay, and I'll publish the ISBN example.