This site is a repository for interesting mathematical diversions. I'll add a new item each month (or so)—something more involved than a simple puzzle. There are now sixteen topics in this site and they are very diverse. I try to choose "real" problems from everyday life. Contributions are welcome (both new problems and solutions to problems that I post).

Spot it!

We recently purchased a game called Spot it! for a young friend of ours. The game consists of a set of cards with eight pictures on each card. If you choose any two cards from the set, exactly one picture will be found on both cards.

Ignoring the degenerate case, what is the largest possible set of cards?