This is definitely an adult experiment for children to observe, but really cool.
We tried 2 ways to get the egg into the bottle.
1. Put boiling water into a flask.
Put the egg on the flask and allow to cool.
Observe!
2. Light some paper and place into a flask.
As soon as the flame has gone out put the egg on top of the flask.
Observe!
You can even get the egg out!
Make sure the egg is tight against the top of the flask.
Put into a pot of water on the stove and gently heat.
Observe!