Matching Sock Problem
10 green socks and 10 yellow socks are mixed up in your sock drawer.
When you are reaching for your socks in the dark and cannot see, how many socks must you take out so that you can guarantee that you have a pair?
You must take out a minimum of three socks. (If you take out three socks, you can only have a maximum of two different colours, so one colour must be repeated and you must have a pair!)