For added experimentation: Pour off the used vinegar, wash the container, and refill with fresh vinegar. Add the “rubber eggs” to the fresh vinegar and set aside. After about a week’s time, remove the egg from the vinegar and note how the size of the egg has changed.
Tip: This experiment can also be done with chicken bones. Follow the instructions as above, leaving the bones in the vinegar for 1 to 2 weeks. The calcium carbonate in the bones dissolves, leaving behind mostly collagen and other connective tissue that is flexible and rubbery.