Here is a classic puzzle. Say that you have two hourglasses of different sizes. The bigger hourglass runs for 11 minutes while the smaller one runs for 7 minutes. Using these two hourglasses, how can you measure 15 minutes exactly.?
Step 1: Start the two hourglasses at the same time.
Step 2: When the 7-minute hourglass finishes, quickly turn it over without wasting a millisecond. At this point, 7 minutes had gone by.
Step 3: When the 11-minute hourglass runs out, the bottom part of the 7-minute hourglass has 4 minutes worth of sand. Now, quickly turn over the 4-minute hourglass. At this point, 11 minutes had gone by.
Step 4: Just wait for the turned over 7-minute hourglass to finish its 4 minutes worth of sand. Once it runs out, exactly 15 minutes will have gone by assuming that you turned over the hourglasses at a nigh-superhuman speed.
This is the only optimal solution for solving this puzzle. Well, you could always buy a clock or a watch I suppose.