It doesn’t take a science major to know eggshells are fragile. So it’s a wonder they survive having the hens sit on them. You might think even with the mother hen’s best intentions, their eggs would produce instant omelets, not new chickens. The main safety precaution is the hen is squatting, not sitting. Her full weight is not on the egg. Also, her body is contoured to fit just right over the egg, surrounding it with warmth without crushing it.