The Two Strings Puzzle

Two strings (each several inches long, but not the same length) have been constructed in such a way that even though they are not the same length, they will each burn for one minute. The burn rates are not steady (thus, you cannot cut them in half and get a 30-second burn). You have the two strings and all the matches you need. Your mission, should you decide to accept it, is to build a 15-second timer...

The answer...