A talking bottle: What happens when air is heated?

We can get a bottle to talk to us:

Here's the trick:

Fill a bottle with very cold water.  Put the (wet) lid upside down on the bottle.

Then get someone with really warm hands to hold the bottle.

The lid will pop up and down as if talking to us.

Why does this happen?

The cold air in the bottle heats up when you put warm hands beside it.

Warm air takes more space and rises, pushing the top off the bottle.

Some of this air escapes and the lid falls down again, but as you heat up more of the air in the bottle, the lid continues to pop up and down.

This is a really fun investigation.