I have just been sent a little puzzle called a So Doku. Not a mathematical puzzle dispite it's appearance. It is quite addictive. There is an example of such a puzzle from here. Scroll down, there is an easy, medium, hard and very hard one.
The rules are easy, each row has to contain the digits 1-9 once and once only, each column 1-9 once and once only and each 3x3 box 1-9 omce and once only.
I hope you all waste as much time on this as I did.