Just place an egg (pretty sure you can do raw or boiled) in a container and cover it with vinegar. After 24 hours dump all the vinegar out and cover it with vinegar again. How long you leave it in the container is up to you. We left ours for 2 days.
Look at the egg! The shell is gone and all that's left is the membrane, so neat! It was soft and squishy, rubbery and bouncy and we could see the yoke inside.
Be sure to give this simple activity a try with your kids, they'll love it!