Here's a good riddle from Brian Brushwood of Scam Nation. He found it in Book of Riddles by Fabrice Mazza and Sylvain Lhullier
Imagine that you're making a magic potion. You're a wizard with a long beard. But — the potion only works if you wait exactly 45 minutes before you stir it. If you stir it before or after the potion's totally ruined. You don't have a smartphone. You don't have a watch. You don't have any kind of time measuring device. What you have is two fuses of irregular consistency. The one thing you know for a fact is that it takes an hour for each of these fuses to burn from one end to the other. How do you use these to measure exactly 45 minutes?
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I'm still trying to figure this one out, so I'm not looking at the comments yet!