The billiards are bulging

One day, Xiao Gang and Xiao Li played billiards. The ball rolled to the ground accidentally, and Xiaogang did not see the ball flattened. The billiards are flat, but there is no ball, and you can't play.

How to do it? After a while, Xiaoli came up with a good idea. She took Xiaogang to the kitchen. Xiaoli took a pot of boiling water and put the billiard into the cup. After a while, the billiards slowly bulged.

They said happily: "You can continue playing."