I had always heard that if you were in a group of people and asked each of them for their birthday, depending on the number of people present, you could probably find two people with the same birthday and maybe even someone with your birthday. It was a good party game.
I decided to look up information about it and found this on Wikipedia:
A probability theory, or birthday paradox concerns the probability that, in a set of n randomly chosen people, some pair of them will have the same birthday. By the pigeonhole principle, the probability reaches 100% when the number of people reaches 367 (since there are 366 possible birthdays, including February 29).
However, 99.9% probability is reached with just 70 people, and 50% probability with 23 people. These conclusions include the assumption that each day of the year (except February 29) is equally probable for a birthday. The history of the problem is obscure.
A graph showing the computed probability of at least two people sharing a birthday amongst a certain number of people:
I just think this is very interesting. Try it at your next gathering and let me know if it works!