system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't animals with gills breathe air?
gpt
Fish gills are very delicate, and have a very large surface area (which makes them work so well). It is completely dependent on being immersed in water to support their weight. Out of the water, the delicate gills will collapse like wet tissue paper, and very little surface area is left exposed, so gases can not be properly exchanged. Therefore, most fish can't survive out of the water for a long time, because oxygen deficiency will catch up with them and they asphyxiate. 

If we could find a way to keep the gills supported and moist without being immersed, a fish could survive for a lot longer, but this it isn't physically possible, even in a humid air-filled chamber at zero gravity, considering that the gills would just stick to one another. Water must completely fill the gill chamber to keep everything else in working order.