There is insufficient information in the question to answer it. How are the ping-pong balls hanging? By what? Are they touching? What is the orientation of the straw with respect to the ping-pong balls? Please restate the question.
Yes, but it's not under pressure or anything. They just happen to be hollow.
Nope, just plain old air. Although they smell weird on the inside, it is only air.
the ball is the same as a tennis ball. it is made out of air on the inside so it will float in the air for a while and then will endventualuy fall.
It contains some oxygen yes. 78% Nitrogen, 21% Oxygen, ~1% Argon plus a smidge of CO2 and other trace gases.
No ... the inside of a ping pong ball is filled with air. Click on the 'Inside of a Ping Pong Ball' link below to learn more.
The heat from the boiling water warms up the air inside the ping pong causing the air to expand and push the dent of the ping pong back out.
Mostly air.
The interior of a ping-pong ball is filled with air, while the interior of a golf ball is made of solid material (rubbers) that are heavier (denser) than air. Thus wile about the same size a golf ball is much heavier than a ping-pong ball.
Its air inside contracts
You can demonstrate Bernoulli's principle using a hair dryer and one or two Ping-Pong balls. Simply turn on the hair dryer and point it upwards. Then place the Ping-Pong ball in the air stream so it the upward force of the hair dryer balances the force of gravity. The ball will appear to levitate. This demonstrates the Bernoulli principle that fast moving fluids are at a lower pressure than slow moving fluids. Air from the hair dryer is at a lower pressure than the outside air. So a ball that is smaller than the air stream can be balanced in it.
The world record for continuous ping pong ball bounces in the air is 8,214 times, achieved by Ashrita Furman in New York on June 12, 2010.