People wear me, yet I'm all alone. People play me with all manner of royalty, yet they're all alone. What am I?
You have 100 coins laying flat on a table, each with a head side and a tail side. 10 of them are heads up, 90 are tails up. You can't feel, see or in any other way find out which side is up. Split the coins into two piles such that there are the same number of heads facing up in each pile. How did you do it?