According to principle of heat-expansion and cold-contraction，to restore it;
Firstly, you need to prepare a water container and hot water with the temperature above 80 degrees.
And then, put the flat ping-pong ball in the container with the hot water.
Finally, let it sit for five minutes and take it out, and the ball comes back.
Note: Remember hot water above 80 degrees; The resting time can also be extended according to the degree of flattening of the ball.