Yes bones are hard.
Bones are made of collagen which is a protein that makes the bones hard.
Along with the collagen protein also contains calcium phosphate that makes the framework of the bones hard and strong.
The bones naturally release the calcium when needed into the bloodstream.
That's why we need to have enough calcium in our bodies to help keep our bodies and bones strong and healthy.
Milk contains the calcium just like eggs also contain calcium which helps keep our bones strong and healthy and helps our bodies grow.