500
You are getting dressed in the dark and realize that you forgot to bind all your socks together into pairs. However you know there are exactly 10 pairs of white socks and 10 pairs of black socks in your draw. All the socks are exactly the same except for their color. How many socks do you need to take with you to ensure you have at least a pair that match?
What is 3?
If you pull out just two socks, it is possible you will have one black sock and one white sock. However, if you pull out a third you are guaranteed to have a matching pair- either with three of the same color of one odd one out.