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.
I Can Be Hot, I Can Be Cold
Riddle:
I can be hot, I can be cold,
I can run and I can be still,
I can be hard and I can be soft.
What am I?
Two sisters we are, one is dark and one is fair,
In twin towers dwelling we're quite the pair,
One from land and one from the sea,
Tell us truly, who are we?