What is a ping pong ball filled with?

Ping pong balls are filled with air. The International Table Tennis Federation has strict rules on the production of table tennis balls, including rules for size, weight and hardness.

Table tennis balls are flammable, and because of this, some people may assume they are filled with something other than regular air. They catch on fire so easily because they are made of celluloid, the same material that comprises photographic film. The process of creating a table tennis ball involves alcohol adhesive. A burning table tennis ball also gives off a chemical odor, but this is from the composition of the actual ball burning, not from the contents.