Milk-Bones are "bone" shaped biscuits for dogs, marketed to pet owners who want to keep their dogs' teeth healthy while dispensing a treat at the same time. The biscuits are mostly cornmeal/wheat flour, contain calcium (and other vitamins & minerals), come in a tartar control variety, and are manufactured by Nabisco.

Coincidentally, Nabisco also manufactures Zwieback, which tastes just like Milk-Bones, just a little sugary - perhaps this is why kids gnaw on them? I know I was happiest at age three with a large sized Milk-Bone in my maw.