The Thread Problem: You have two threads. Each of them will burn for exactly one hour if one of its sides is set on fire.
Question: How can one measure 45 minutes exactly, using only these threads and a lighter ?

Answer: Fold one of the threads in half and light both loose ends(not the folded end). This should burn for 30 minutes. While that's burning, fold the other thread in fourths. As soon as the first folded thread is burnt out, light the other folded thread on one end. This should burn for 15 minutes. So the 30-minute burn plus the 15-minute burn equals a 45-minute burn.