Math Puzzle

For Friday Sept. 13th. Find and master a Math Puzzle.
Present it in class. Be able to explain how it is solved.
Try to keep it at the level of your classmates.
If it requires a printout, please make two copies.
One with your explanation (to hand in), and one to give to a partner.

Possible sites: http://mathriddles.williams.edu/

http://mathforum.org/isaac/mathhist.html

http://www.rinkworks.com/brainfood/p/math1.shtml

http://www.barcodesinc.com/articles/math-logic-problems.htm