You have to boil a single egg for exactly nine minutes.
All you have are two hourglasses, able to measure (in sand) seven and four minutes, respectively.
How quickly, using only these two hourglasses, can you boil an egg?
First you let run both clocks. the 7 minute clock would run until it finishes while the 4 minute clock would be immediately turned the other way around after the 4 minutes and kept for the remaining 3 minutes of the duration of the 7 minute clock.After that 3 minutes the clocks are turned around and the seven minute clock will run until the 4 minute one will stop (this is for 1 minute since that was the sand left in the 4 minute clock).At this point we would have the first 7 minutes + the 1 minute = 8 minutes. By turning the 7 minute clock around once again (It now has one minutes worth of sand) we will get the other minute required which will get us 9 minutes. We can now serve the boiled egg.