I came across the “send more money” puzzle in a worksheet for 11 year olds. It took me a good 15 min to crack it but it was fun. But 11 year olds? I guess there are many very smart kids around. I’ll see when I try to teach the enrichment class next month.
The linked site is a very nice page containing many more puzzles for you to crack your head against. This reminds me of the first puzzle of the google aptitude test, I still have not managed to solve it completely by hand. (Yes, I’m aware of answers available on the web.)