Riddle: You are given 2 fuses that burn for exactly one hour. Measure exactly 45 minutes. The fuses burn unevenly so you can't measure the time by the length of the fuse.
Answer: If you ignite simultaneously both ends of one string and just one end of the other, in half an hour the first one will have burned completely up, and the other will have a half hour left. Light the other end of that one, and it will take just 15 more minutes to finish up. Abracadabra!