Question: Why don't fish get water in their eyes? (from reader zaytwzatar's 5-year-old nephew)
Answer: Fish live in lakes, rivers and oceans, and water comes into contact with their eyeballs all of the time.
Fish do get water in their eyes. Not behind the eyeballs, of course, but water does touch their eyeballs.
"They don't have eyelids," said fish physiology expert Richard Brill, affiliate faculty at the Virginia Institute of Marine Science in Gloucester Point, Va.
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That means fish don't blink or close their eyes. Ever. The only exception is a shark, which has a nictitating membrane that works something like an eyelid. The membrane covers the shark's eyes when it feeds.
So, does getting water on their eyeballs hurt them? Not at all, said Brill. "The function of the eye evolved to suit the habitat." Just like people don't mind getting air on their eyeballs, fish don't mind getting water on their eyeballs.
In fact, the structure of a fish's eyeball is almost identical to a human's eye, except for the lens. This is the part behind the iris that focuses light onto the retina. Because humans live in an airy environment, our lens is mostly flat and thin. But because a fish lives in the water, where there is less sunlight compared to above the water, a fish lens is thick and shaped like a marble. if a human opens his eyes under water, the world appears blurry. That's because our lens works better in the air. But a fish sees clearly under the water.
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Swordfish (pictured above) have remarkably large eyeballs. An adult swordfish about six feet long, for example, could have an eyeball as large as a softball. These fish have big eyes because they hunt squid at ocean depths between 500 and 700 feet. It's dark and cold at these depths and having a large eye helps them see better in the dark. The fish also have special muscles around their eyes designed to keep the retinas warm in the cold water - this helps them see fast-moving prey better.
So, fish do get water in their eyes, but they don't mind. It's all part of life underwater. Humans that get water in their eyes might find it uncomfortable or even painful if the water contains soap or chlorine. Best to wear goggles in the pool.
Credit: Franco Banfi / Getty Images This blog is part of a series "Kids' Questions Answered," where we consult the experts to find the best answers possible to children's confounding queries.