1. Fish suck oxygen from the water through four gills in the gill cavity, and then discharge carbon dioxide from the body into the water one by one through the gill cover.

2. When the fish leaves the water, the water will not flow to the gill cavity, and the gills will soon dry up, and the gills will stick together, so the fish can no longer breathe with the gills.

Without breathing, fish will suffocate to death. So fish spend their whole lives in the water, eating in the water, sleeping in the water and swimming freely in the water.