You have only two hourglasses. The first one times exactly 7 minutes, the other times exactly 11 minutes.
Using just these two hourglasses and nothing else, how would you accurately time 15 minutes?
A bit tricky this one, the solution is to:
1. Start both hourglasses at the same time.
2. At the end of 7 minutes, quickly flip over the 7-minute hourglass.
3. In another 4 minutes, the 11-minute hourglass will be empty and 4 minutes will have elapsed from the 7-minute-hourglass. Thus, once the 11-minute hourglass has been empty, quickly flip the 7-minute hourglass over and watch the remaining 4 minutes fall down.
This gives: 7 minutes + 4 minutes + 4 minutes = 15 minutes. 🙂