system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it easy to open eyes underwater, but hard to keep them open when you go up?

You keep your eyes open underwater no problem, the moment you go up it's hard to keep them open without clearing the water around the eyes. Why?
gpt
When you dive underwater and open your eyes, your eyes lose the mucous that's supposed to keep the moisture in. It might sting due to extra chemicals in the water (chlorine in a pool, salt in the ocean) but the water alone is perfectly fine for your eyes. Once you surface the water will fall off and your eyes are now completely exposed to air without their protective mucous. It makes your sensitive eyes very dry.