arithmetic
How to write down unimaginably large numbers using just a few symbols. 

Why do we add fractions the way we do? A very close look at addition reveals the answer — and it also works for negative numbers. 
The Indian way of adding fractions makes things easier (with musical explanation). 
How would you go about adding up all the integers from 1 to 100? Tap them into a calculator? Write a little computer code? Or look up the general formula for summing integers? 
The number 1 can be written as a sum of unit fractions, that is fractions with 1 in the numerator. But how long can we make such a sum? 
On March 14 2010 a mathematician and a magician teamed up to perform what they believed to be the world's largest live magic trick. The trick involved a thousand volunteers from the around the world who, using free choice, each came up with a number that was only known to themselves. And although the volunteer might be on the other side of the globe, the mathematician and the magician were able to read their mind and tell them which number they had chosen. 
John Napier was a clever man indeed. Besides inventing the logarithm, he developed ingenious calculating devices that fully exploit the power of the positional system. In this article Chris Sangwin tells you how to make your own set of Napier's bones and perform mathemagic with an interactive checker board.

Runner up in the schools category Being good at mental arithmetic isn't going to gain you much street cred these days. But, as Owen Daniel explains, not so long ago it was a sure route to fame and fortune — even if you were a horse.
