When mom cooks eggs, she doesn’t throw the eggshells away, she will crush them up and feed them back to the chickens because it provides calcium that the chickens need.
Mom’s Daughter: “Why do you do that?”
Mom: “… so I don’t waste nothing…”
The hen’s diet has to include a certain amount of shell material so she can lay hard eggs (hard shells). If the hen lacks this in her diet, she may lay “rubber” eggs, or eggs without shells. The outside is a thick membrane around the egg, and when you pick it up it feels like droopy rubber. It’s quite weird.
So to help the hens not lay rubber eggs, Mom adds oyster shells to the chicken’s over-all diet as a calcium supplement. It strengthens the egg shell.
In addition to the oyster shell, Mom will just feed the shells back to the chickens. It seems quite resourceful, don’t you think? This is such a popular topic, there’s a wiki page on how to do it: