A supposedly wise ruler asked a servant what he’d like as a reward for an act of bravery. The servant said he’d like a grain of rice on the first of the 64 squares on the emperor’s chessboard, two on the second, four on the third… doubling the number of grains for each square. By the 64^th square he’d have accumulated 2⁶⁴-1 grains (18,446,744,073,551,615) or more than a thousand years’ worth of the world’s rice production.