HelloCustomer and thank you for your question.

Ping pong balls are made of celluloid, which is the same material from which photographic film is made. No, it is not considered a true plastic. Celluloid is a composition of nitrocellulose and camphor.

A flat sheet of celluloid is soaked in hot alcohol until soft. It is pressed into a hemispherical mold, trimmed along the edges, and allowed to harden. The hemispherical mold produces a half a ball. Two halves are matched together to produce one whole ball. The two halves must be exactly the same weight. The halves are perfectly aligned and glued with an alcohol-based adhesive. The balls are then put into a machine that agitates them to smooth the seams. The finished balls should not have seams that can be seen or felt. The grading of ping pong balls depends on the smoothness of the seams, and if the seams can be felt then the ball is not suitable for play and will usually be sold for other purposes.

Ping pong balls are filled with air. But because of the alcohol-based adhesives, and the materials of which the ball is made, if you were to pop a ping pong ball and breathe that air [NOTE THAT I DO NOT RECOMMEND THIS] it would smell of chemicals (think of the smell of photographic film, combined with model glue). This seems to be one reason that many people think that ping pong balls are filled with a gas other than air.
Ping pong balls CAN be filled with virtually any gas - much in the way that a balloon could be - but your standard ping pong ball is filled with air.


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The downside to lighting a ping pong ball on fire in a classroom is that a ping pong ball is highly flammable, and the materials of the ball itself and the adhesive that is used to put the halves together are both things that can produce noxious fumes.
Also, the celluloid material of the ball could cause some nasty burns if it happened to drip onto someone's skin.
In general, burning a ping pong ball would be similar to burning photographic film - both in terms of the dangerously hot, sticky, residue it would create, and the fumes produced.
I certainly would not recommend burning a ping pong ball in a classroom.