system
Given the following post, respond as if you're commenting on Reddit:
human
I get it; fish can’t breathe out of water, but WHY?

To be more specific… I tried looking for answers online, but only found things talking about the fish needing water to pass over their gills to extract the oxygen from the water. Okay, but why? Why can fish not breathe in open air?

There is oxygen in the air and yet they can only utilize it from the water? Is it that fish actually *can* breathe out of water, but their gills “collapse” or “flatten” (can’t think of a better way to describe it) without being submerged in water and make it too difficult/laborsome to do so? (Like reduced surface area/capacity of the gills)
gpt
You know how, when underwater, a person’s hair can just kinda float freely, each individual hair flowing separately from every other…but when they come out of the water, their hair flattens against their head and sticks together in thick clumps?

A fish’s gills are kinda like lots of thin filaments. In the water, they float apart and water can flow between them, but come out of the water and their gill filaments clump together like so many wet hairs. This means most of their gill surface area is packed inside other filaments, with too little still-exposed surface area to oxygenate their blood as much as they need to live.