This article is from the Puzzles FAQ, by Chris Cole firstname.lastname@example.org and Matthew Daly email@example.com with numerous contributions by others.
Every morning when I warm my milk for breakfast, I put one cup of milk
in the microwave (which is in working order) for exactly 84 seconds. Why?
When you put your cup in the microwave, the handle on the cup is
pointing towards you. Your microwave has a revolving plate on which the
cup is placed. After 84 seconds, the handle is again in it's initial
position, so you can take the cup out without burning your hands.