After cooking a whole chicken, I had a wishbone to show Bear. I told her about how you can wish on this bone when you break it. Since we then had two pieces, I put one in vinegar and left one out in the air. I told Bear she’d feel something really neat in a two or three days.
The bone left in the air was hard and stiff.
The bone in the vinegar was all rubbery.
The vinegar is a mild acid and dissolves the calcium in the bone. Calcium is what make the bones hard.
Here’s a link to a detailed how to for those who like directions.