Here’s an interesting experiment. We promise you’ll be talking about this one at school the next day. Find a bottle with a long, narrow neck and set it on the table. The opening should be just small enough to keep the egg from falling inside.
One of the basic physics principles is that heat causes things to expand (get bigger) and cold causes things to contract (get smaller). Things such as air will contract and take up less room when cooled. Similarly, things will expand when they get hot.