You have a piece of string which when set on fire from one end takes 120 minutes to burn completely. It has a property that the time taken to burn a distance D from one end is the same as the time taken to burn a distance D from the other end. The rate of burning is not uniform – different parts of the string will take different amounts of time to burn.
You need to boil an egg for exactly 15 minutes using the string as a timer. How will you do it?