A recipe requires 15 minutes to cook. You have a 7 minutes hourglass and an 11 minutes hourglass with you. How will you measure 15 minutes using them?
Remember what happens when you turn an hourglass, it doesn't start from the beginning but from where it got to...
You turn the 7 minute one and the 11 minutes one at the same time. When the 7 one finishes, turn it again. When the 11 one finishes, you know that 4 minutes have passed, with 4 minutes to get to the desired 15, all you need to do now is turn the 7 minute hourglass again since it counted 4 minutes, and when it runs out it'll be exactly 15 minutes.