Answer:
It is the great nemesis of the lactose intolerant.
What is full of holes, but can still hold a lot of water?
What surrounds the world, yet dwells within a thimble?
What has a head, a tail, is brown, and has no legs?
When it is alive we sing, when it is dead we clap our hands. What is it?
You use it between your head and your toes, the more it works the thinner it grows.