All you need to make a bouncy egg is a glass, an egg, and vinegar. Fill the glass with vinegar, drop in the egg, and wait for the vinegar to dissolve the shell. The process starts immediately, and it will take about 24 hours to complete. (If the shell isn't completely dissolved after 24 hours, change the vinegar and let it sit for another 24 hours.)
After the shell has completely dissolved, rinse the egg in cold water, and dry it off. Children will be amazed by the "rubber" egg they have created.
This really is a fun way to explore science with children. My boys loved playing with the bouncy egg, shining a flashlight into it to see the yolk, and poking it with a knife to see the inside. Is there anything better than feeding the curiosity of a young mind?