In The Big Bang Theory episode "The 43 Peculiarity", Howard and Raj try to solve the mystery of Sheldon disappearing every afternoon to a room with a chalkboard that has the number 43 written on it. While Raj and Howard thought Sheldon has discovered a parallel universe (via a fake video), the viewer learns 43 really stands for the number of times Sheldon could bounce one of these (but had yet to replicate since).
Hacky sack