And I think I've worked out a way to measure the radius of a perfectly round pool using just a perfectly round rock...

When you drop a rock into water it creates ripples throughout the water, the very first ripple will only be the size of the rock as it creates that when it enters, the second ripple is created from the water rushing back in too fill this hole and then more ripples are created from the force of the second ones. All of the ripples are dependant on the size of the stone as they all originate from one point.

Assuming that this is true and then that all the other ripples are therefore relative, you should be able to measure the increase in the gap between each ripple.

Assuming the gap of the ripple gets larger by 1.1 squared each time then and there are 3 ripples the radius of the pool should be (((1.1 squared)squared)squared)

This is my mind on drugs