system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do bones grow?

Tonight my son (3 1/2) was playing with his 9m old sister and noticed that his arms were longer. He asked why and I said he was older and had grown more. Then, because I apparently didn't feel dumb and caught off-gaurd enough in his eyes he said, "How do my bones grow?" I told him I didn't know, would ask some very nice internet people, and get back to him.

And before the 8 million "bone" jokes roll in, I'll be enjoying those myself.

Thanks in advance!
gpt
Your bones start off soft when you're a baby and grow stronger and longer as you get older. There are parts of your bones that stay soft a little longer (growth plates), and thats where the new bone comes in, and when it closes all the soft parts, you stop growing!