Contrary to popular myth, table tennis or "ping-pong" balls are hollow and contain only regular air on the inside. However, when put to a lighter or flame, ping-pong balls ignite and burst into flame.
Standard ping-pong balls measure 40 millimeters in diameter and weigh 2.7 grams. Ping-pong balls have a matte finish and are made of celluloid plastic, which is flammable. This is the actual reason for the explosion. They are less dangerous to burn than older ping-pong balls, which consisted of acidified celluloid that became increasingly unstable over time; the slightest spark or heat from friction often ignited the balls.