Explosion in a bottle

Throw a burning piece of paper into an empty milk bottle and stretch a piece of balloon rubber firmly over the mouth.

After a few moments, the rubber is sucked into the neck of the bottle and the flame goes out.

During combustion, part of the expanded, hot air escapes. After the flame goes out the diluted gas in the bottle cools and is compressed by the external pressure. The rubber is therefore stretched so much that the final pressure equalization only occurs if you break the bubble, causing a loud pop.