Boiling an egg  logic puzzle
Suppose you would like to boil an egg for exactly 15 minutes. But you only have two hourglasses, one of zeven and one of eleven minutes. What is the the fastest way to do this? 
Explanation
Turn both hourglasses upside down and put the egg in the water. After 7 minutes, turn the 7minutes hourglass. Then when 11 minutes have passed, turn the 7minutes hourglass again. Then it will take another 4 minutes until the 7minutes hourglass stops. At that time exaclty 15 minutes have passed.
