system
Given the following post, respond as if you're commenting on Reddit:
human
Does the earth "lose/gain" water or is it truly a closed system?
gpt
The Earth doesn’t lose much water, as the ozone layer prevents UV light from breaking water molecules down to hydrogen and oxygen, the mass of the Earth prevents water vapor from leaving the atmosphere and the vapor condenses and falls back to the surface. 

Earth also gains a marginal amount of water from asteroids and the like. 

So it technically isn’t a closed system, but the rate of change isn’t significant.