A cook must put a souffle exactly 9 minutes at the oven, but he only have a 7 minutes hourglass and a 4 minutes hourglass.
How can he measure 9 minutes with those clocks?
Turn at the same time both hourglasses. When the 4 min hourglass finish, turn it again. When the 7 min. hourglass finish, turn it. At this time 8 min. have pass and only one minute is needed. When the 4 min. hourglass finish, turn again the 7 min. hourglass. The sand that have fall is going to last one min. and you have count 9 min. in total.