It happens very often when playing a ping pong and it's very easy to crash the ball.
If that happens, are there any easy tricks to fix the ball?
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similar to @Duncans answer, you can pass a lighter flame over the ball a few times. it will heat up the air inside and pop out the dent. be careful not to hold the lighter on the ball for too long or else it will melt the plastic. Also, it should be noted that this method and @Duncan's will not work if the ball is dented to the point that the plastic has cracked and allows air to pass between the inside and the outside.
I'd also suggest just to use a new ball but in a pinch this will work
Source: beer pong in college
If you can reduce the pressure outside the ball without heating the ball, the ball may suffer less damage. You could put the ball in a container and suck. This will be more effective with a small container. You could also try using the suck end of a vacuum cleaner (if suck ends still exist - haven't seen one for a while), or take the ball to high altitude, or use a plunger, or make (or redeploy) some kind of piston. A vacuum chamber might do the trick. http://en.wikipedia.org/wiki/Vacuum_chamber