This experiment takes some days to accomplish, but it is worth the wait.
Use any leftover bones from your meal. A chicken leg is perfect, but other bones will work too. Remove all meat, gristle, and skin. Completely cover the bone in vinegar in a glass jar. Attach the lid and set aside.
Check the bone after three days, five days, and seven days. It should progressively become more pliable so that you can bend it in half without breaking it.
You can use either a fresh egg or a hard boiled egg for this experiment. In fact, you can simply use eggshells themselves and bypass the entire egg.
1. Immerse the egg totally in vinegar and let it sit.
2. After one day, check the egg. Look for bubbles on the shell. Those are carbon dioxide bubble that the reaction creates. Feel the eggshell to see that it is changing.
3. After three days, check the egg again. By seven days, the eggshell should be completely dissolved.
Why it Works
Bones and eggshells are hard because of the calcium content in them. Vinegar is an acid (acetic acid) which dissolves calcium. After soaking in the vinegar, the calcium from the bone is dissolved away so that all that is left is the soft tissue which is pliable like rubber.
Eggshells are 94% calcium carbonate, so after the vinegar dissolves the calcium, there is little left except the thin membrane that protects the egg.