A fun probability bet.

[marbles]

I remember this one from when I first started learning about probability. You might want to try it out sometime:

You're in a group of 20 people. No one knows anyone else's birthday, but it is widely known that none of them were born on February 29th. Your friend offers to bet you that no two people have the same birthday (month and date only). Hungry for action, he even offers you 3-to-1 odds. Should you take the bet?