What's the coolest mathematical fact that you know?

I dunno about magic, and my math knowledge stops at quadratic equations — hell, musicians never have to count higher than 3 — but 121 (eleven squared) is a curious number in which to divide integers. Numbers 1 through 9, when divided into 121 all give infinitely repeating cycles of 22 digits, with patterns ever more curious as you examine them. For example, 1/121=
.00826442680991735537190082...
You list all the quotients from 10/121 down to 1/121, and you find staggered mirror images spreading up and down from these two:
6/121 = .0495867768595041322314049...
5/121 = .0413223140495867768595041...