Birthday Paradox
There is a odd thing that happens when you have 23 students in a classroom. It is quite possible that at least one of them has the same birthday as another (in fact the probability is about 50%). This is called the Birthday Paradox.
This concept has parallels in cryptography, where you are counting on two people to not hold the same random numbers. I ran across a a little calculator for these kind of events:
Perhaps this is a little too “Number Theory geeky,” but I find this calculator quite cool and useful.