One day a little girl put a nail in a stem of the tree to mark her height. She has aged 10 years and comes back, but sees that the height hasn't changed. Why is this so?

The tree has been growing as well or she has reached her final height ten years ago.

The tree has been growing as well or she has reached her final height ten years ago.

 

 

I suppose those are logical answers, but people can still change their height even after they become and adult, like perhaps because of surgery, hair length, or maybe the shoes she wore that day, and even shrinking as you get older.

I would say though that there is a more probably answer than that the tree has been growing at the same height as her. In fact, the tree's height changing doesn't have much to do with the position of the nail.

To a fair approximation threes grow from the tips of their branches. The trunks get thicker but not much taller.

To a fair approximation threes grow from the tips of their branches. The trunks get thicker but not much taller.

 

Right on.