Questions kids askLiving things

How do fish breathe underwater?

Fish breathe with gills, which take the oxygen dissolved in water and pass it into the fish’s blood as the water flows over them.

How to explain it, by age

This is how Pukki answers the question, once for each of the five age bands it speaks to. Pukki is a voice-first AI encyclopedia for kids: a child asks out loud, and it gives a short spoken answer made for their age.

For a 3- or 4-year-old

Fish breathe with gills. Gills take tiny bits of air from water as it passes over them, like a little air filter.

For a 5- or 6-year-old

Fish breathe underwater with gills, because gills take oxygen from the water. Water flows over the gills, and tiny blood vessels collect the oxygen, like lungs collecting oxygen from air.

For a 7- or 8-year-old

Fish breathe underwater using gills. Water flows over the gills, and the gills take oxygen from the water into the fish’s blood, like tiny air-catching screens. The water touching the gills soon gives up its oxygen, so fish keep fresh water moving through their gills.

Edited. Pukki said “Fish cannot use the oxygen in water forever, so they need water moving through their gills.” That sentence is muddled. What it means: it is the movement of water across the gills that lets oxygen pass into the blood, so the water has to keep flowing. (U.S. National Park Service)

For a 9- or 10-year-old

Fish breathe underwater using gills. Water flows over the gills, and tiny blood vessels inside them take oxygen from the water into the fish’s blood, while carbon dioxide moves out. Gills work a bit like a tree’s leaves, but underwater.

For an 11- or 12-year-old

Fish breathe underwater using gills. Water flows over the gills, and tiny blood vessels inside them take in oxygen dissolved in the water, while carbon dioxide moves out. Gills work a bit like many thin, wet filters, giving the fish lots of surface area for this exchange.

A little more, for you

Water holds a little dissolved oxygen, far less than air does. Gills are made of paper-thin tissue full of blood. Water comes in through the fish’s mouth and flows out over the gills, and on the way oxygen passes from the water into the blood.

Fast-moving water, such as a mountain stream, holds more dissolved oxygen than still water.

Questions to ask them

Where have you seen a fish?
Did you see its mouth or the sides of its head moving?
What do you think would happen if the water stopped moving over its gills?

These two parts are ours. In the app, Pukki writes its own for every answer, under “Explain it to me” and “Ask them back”.

Try it together

Watch a fish moveLook for a fish’s gill covers gently opening and closing.

Pukki’s own idea, given to the parent with one of the answers above.

One more why

Where these answers come from

We asked Pukki this question ourselves, once for each age, on 2 October 2026. No child’s question is published here. The answers are written by AI, and we read each one against the sources below. One of the five is marked Edited, with what we changed and why.

Pukki’s answers vary from one asking to the next, and can be wrong. It answers in the English its device is set to; these were asked in American English. Today Pukki also ends most answers with a short question on the same topic, which a parent can turn off.

Read against these sources on 5 October 2026.