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.
VIDEO: Flushed Fish Invading Oceans
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.