Suppose you are given many pieces of string which burn from one end to the other in exactly one hour. The problem is that they are not identical and do not burn uniformly down their length, so if you light them at one end, then half the string does not burn in half hour. How can you use these strings to measure a 15-minute interval?
Solution: Take two strings. Light one at both ends and simultaneously light the other at one end. When the first one burns out it will be exactly 1/2 hour. Stop the other one burning at this point. Now the second string will burn through in exactly 1/2 hour. If you light it at both ends it will burn through in 15 minutes. The real question is: how can one make such strings that burn through in one hour?