system
Given the following post, respond as if you're commenting on Reddit:
human
Why are fish unable to breathe oxygen from air when they are able to absorb it from water which is so much more difficult?
gpt
It is because fish use gills in order to obtain oxygen. Which means that the greater the surface area of the gills of the fish, the more the blood vessels are exposed to oxygen.

While outside of water, the gill arches essentially collapse, reducing that surface area to zero. The projections on the gills float in the water in order to allow for the easy passage of oxygen.