Here is a variation on one of Richard Wiseman's Friday puzzles.
For dinner you decide to make a soft boiled ostrich egg. Problem is, an ostrich egg needs to boil for exactly seven minutes. After searching through your kitchen drawers, you find two hour-glass-style egg timers: one for three minutes, the other for five minutes. Is it possible to time exactly seven minutes using these timers? Is it possible to do so without any preparation?