Swimming pool without water.

Jack: You know Dad built three swimming pools in our new house.

Hery: Isn't that so!

Jack: Yes, one swimming pool with hot water, one with cold water, another without water.

Hery: What is that! The one who feels cold will swim in the pool of hot water and the one who feels hot will swim in the pool of cold water, so far I have understood. What is a swimming pool without water for?

Jack: For those who don't know how to swim!