You want to boil a two-minute egg. If you only have a three-minute timer (hourglass), a four-minute timer and a five-minute timer can you boil the egg for only two minutes?

Why this works

This riddle involves strategic timing with the available hourglasses to measure out exactly two minutes. By flipping the three-minute and five-minute timers simultaneously when the water starts boiling, you create a situation where the three-minute timer counts down while the five-minute timer continues to run. When the three-minute timer runs out, you know exactly when to start boiling the egg; then, when the five-minute timer runs out two minutes later, you can confidently take the egg out, having boiled it for precisely the required time. The clever use of the different timer lengths allows you to isolate the two-minute interval needed without directly using a two-minute timer.