Today I was at a job interview and the last question was, "How can I measure 5 minutes by lighting two ropes that each take an hour to burn but do not burn uniformly?" (classical).
I know how to measure 15 minutes and 45 minutes, but 5 minutes....?
Cut the first rope into 12 pieces and light one end of each. Throw the second rope in the trash. If at any time one of the twelve segments is about to be completely consumed, cut some of the extra off of any of the other longer pieces and attach to the nearly-consumed segment, so that all twelve segments finish burning at the exact same time. Then 60 minutes of rope is consumed in 5 minutes.