This is an awesome website for kids' science experiments. How to make an egg shell disappear without touching it. Lots of fun science experiments on this website, along with explanations for the kiddos.
Soak a raw egg in a cup of vinegar for three days and watch how the egg’s shell disappears! This experiment is a great introduction to the acid-base reactions between acidic vinegar and basic calcium (Science Olympiad Bottle Rocket)