ELI5- Why does it hurt when you run water over your eye but the liquid in around doesn’t

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It’s still water isn’t it? Shouldn’t it have the same effect

In: 4

The liquid around, just like your tears, is not pure water. It has some salt (or other components) in it.

The key concept is “osmosis”. For some membranes (like some cell walls), water tries to have the same concentration of stuff in it (like sugar, salt) on both side of the membrane.

Otherwise, water from the less salted side will move to the more salted side. If a lot of water goes inside the cell or leave the cell, it’s bad for it. That’s why salt kills slugs (and hurts on a wound).

For your eyes, pure water will hurt, and water with too much salt (sea water for example) will hurt as well. But just the good amount will not hurt, and that’s typically what you’ll have in eye drops.

The liquid in your eyes is not pure water. It is salt water with some mucus in it. You can actually taste how salt it is and feel that it is a bit sticky. This mixture feels perfectly normal for your eyes. However if you get pure water in your eyes then it is going to wash away the salt water and make it into fresh water. There is probably also lots of debris in the water which you might not see or feel but your eyes will notice it. The lower salinity and possibly the debris will cause your eyes to hurt. It is basically an evolutionary train to prevent you from doing that.