My goodness, I love riddles too much sometimes. Here's another one I heard from someone.
You want to know when 45 minutes had past. There are no clocks, sundials, hourglass,etc. All you have are two ropes and a lighter. The ropes are different in length, type, and burning rate. However, you know that it takes a total of 1 hour to completely burn each rope. Also both ropes don't have consistent thickness so it might take longer for certain parts of the rope to burn (example: it might take 55 minutes to burn one small section but only 5 minutes to burn the rest, leading to 1 hour total.). How do you find out when 45 minutes has passed?
Hint: you don't need to manipulate the rope in anyway. No cutting, folding, tying, or etc required.
By lighting both ends of a rope, regardless of length, material, or thickness, it will take 30 minutes (half the time) to burn the rope. So burn two ends of rope #1 and one end of rope #2 at the same time. Once rope #1 burns out, quickly start burning the other end of rope #2. By now 30 minutes has passed. The remaining portion of rope #2 should take 30 minutes to burn up. By lighting the other end at this point, it should take only half the time to burn the last portion: 15 minutes. Once that rope finish burning, 45 minutes has passed.
