A famous problem, sometimes attributed to a conversation between a King and his muse. The question is simple - if a chessboard had a single grain on its first square, then two on it's second, four on the third, eight on the fourth etc., then how many grains total would be on the chessboard if every square were filled?

The answer is 18,446,744,073,709,551,615, or 2^64 - 1. This is much, much higher than most people expect it to be at first, so this problem is often used to highlight the enormity of exponential growth and how humans are very poor at comprehending it.