If you read the message regularly, you know I frequently mention my pretend grandson Riley. He’s 18 months old and he has one speed: fast. As a result, he occasionally falls. While sometimes he does fall hard enough to hurt himself, he doesn’t break any bones as I might if I fell that often. Why not?

Most of a baby’s bones begin as cartilage. A baby has about 300 bones at birth. As they grow, their bones begin to get solid; the cartilage cells gradually become bone cells by absorbing minerals such as calcium. Some bones fuse . . .

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