A very nice puzzle from Problem Solving by Thomas DeFranco and Charles Vinsonhaler.
You have two fuses, each twelve inches long. Each fuse burns in exactly one hour, but does not necessarily burn at a uniform rate. Also, the two fuses do not necessarily burn at the same rate over corresponding segments, but a given segment on a given fuse burns in the same amount of time in either direction. How do you use these two fuses to time 15 minutes? (You are allowed as much time to prepare as you wish.)
How do you time 15 minutes using only one fuse?
Problem Solving by Thomas DeFranco and Charles Vinsonhaler.