system
Given the following post, respond as if you're commenting on Reddit:
human
How does the earth make water? Or is there a fixed supply.
gpt
Water is chemically composed of Oxygen and Hydrogen, both of which are found in large quantities on our planet, but not always together as water. The amount of Hydrogen and Oxygen is relatively fixed - discounting the small amount added by meteorites entering our atmosphere, and space craft and the solar wind releasing some back into space.

So there's a maximum amount we could have if all the oxygen and hydrogen on the planet was used to make water, but there would be some hydrogen or oxygen left over, as it's unlikely the mounts match up exactly 2:1. 

Life and human activity constantly moves Hydrogen and Oxygen from one chemical composition to another via chemical reactions: Plants exchange oxygen (both ways) with the atmosphere via photosynthesis and growth; animals breathe in oxygen from the atmosphere and combine it with sugars from food and breathe out carbon dioxide and water vapour produced in respiration. On top of that, the human activity of burning wood and fossil fuels also produces carbon dioxide and water (among other things). Other industrial and natural chemical processes also produce or consume water.