Its a test that will check your IQ.

Suppose you have one 11 minute hourglass and one 13 minute hourglass.

Can you measure exactly fifteen minutes using them ?

First Step: Invert both the hourglasses.

Second step: When the 11 min hourglass is finished, invert it.

Third Step: When the 13 min hourglass is finished, flip the 11 min hourglass would have lost only 2 minutes till now. On inverting, we get to measure those two minutes.

Now 13 + 2 = 15 minutes.

In this way, fifteen minutes can be measured in just 3 steps.