What we did:
In the microwave we heated water until it was boiling. I poured the boiling water into the bottle and let it heat for a moment, then emptied the bottle (an adults job!) and immediately put the cold, peeled, hard-boiled egg on top. Then all you do is watch and wait!
The hot air inside the jug is less dense than the colder outside air. This creates suction which pulls the egg into the bottle. Have some fun and try it! If it worked for us, it'll work for you!!