With a seven-minute hourglass and an eleven-minute hourglass, what is the quickest way to time the boiling of an egg for 15 minutes?
Start both timers as the egg goes into the boiling water.
When the seven-minute hourglass completes, turn it over.
When the eleven-minute hourglass completes, turn the seven-minute hourglass over.
When the seven-minute hourglass completes, take out the egg.