Answer:
What is a five-letter word that has brain in it?
I have rivers without water. Forests without trees. Mountains without rocks. Towns without houses.
They try to beat me, they try in vain. And when I win, I end the pain.
Hold these between your knees if you want to set a Cuban beat.
It cannot be seen, it weighs nothing, but when put into a barrel, it makes it lighter.
You use it between your head and your toes, the more it works the thinner it grows.