If fish use their gills to filter oxygen from water, why can’t they do the same in air?
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Other people are giving you the right answer – it’s about surface area and the gills collapsing. But I feel like that needs a more ELI5 description:
**Picture someone with long straight hair going underwater.** In water, a fish’s gills are [like a person’s hair behaves under water.](https://i.pinimg.com/originals/4f/74/bf/4f74bf522564271a2bca545d32d626f8.jpg) It’s all spread out in the water and every individual strand is floating freely. There’s a lot of hair surface touching the water.
When a fish leaves the water into air, their gills act [like wet hair when you get out of the water.](https://media.gettyimages.com/photos/young-woman-with-long-wet-hair-by-swimming-pool-rear-view-picture-id200244105-001) It all “collapses” into clumps. The amount of contact between hair strands and the surroundings is a tiny % of what it is under water.
Fish rely on their gills being all spread out and free-floating to have enough surface area to pick up enough oxygen. **It’s not that gills are unable to extract oxygen from air and can only get it from water.** The problem is their physical structure prevents them from touching enough air to keep the fish alive.
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