Well, not really string theory, but it's fun to pretend to be able to understand that! This is another from Click and Clack(RIP).
You have two pieces of string, both 4' long. Each string has the property that if you light one end, it will burn up completely in one minute. However, the strings do not burn at a constant or even a predictable rate. I.e., one of the stings may burn up 2' in the first 10 seconds, 1' in the next 30 seconds, and the last foot in 20 seconds. And the other string may burn in a different pattern, just finishing in one minute.
Given those two strings and a zippo lighter, how can you measure 15 seconds?