Brainteaser of the day: If you have a 7-minute hourglass and an 11-minute hourglass, how can you boil an egg in exactly 15 minutes?
Answer: To boil the egg in exactly 15 minutes, follow these four steps. Start both hourglasses as you start boiling the egg. After the 7-minute hourglass runs out, turn it over to start it again. Four minutes later, when the 11-minute hourglass runs out, turn the 7-minute hourglass again. Wait for the 7-minute hourglass to run out, which will take another four minutes and get you to exactly 15 minutes of boiling time.