system
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human
ELI5: When a tree grows, where does the mass for its trunk and branches come from? How does the mass and stuff get to the top to make new tree parts?

Physics has the conservation of mass right?  ... And ... Things like trees sit in the ground. Where do they get the mass to grow tall? How do they move it up their trunk? How do really old trees get more dirt when they make it into trunks?
gpt
> Where do they get the mass to grow tall?

Not from the dirt! The vast majority of the mass which makes up plants including trees comes from the air and water.

Cellulose, the main structure of the tree, is composed of carbon, hydrogen, and oxygen. The carbon and oxygen can come from the air, while water of course contains the hydrogen and more oxygen.

> How do they move it up their trunk?

Water moves up the trunk of the tree via capillary action, water's surface tension, and something called "transpiration". Basically water evaporates from the bottom of leaves and this  acts to pull water into the leaves from the trunk, and by extension all the way from the roots.