system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do polar bears not suffer from snow blindness?
gpt
Polar bears have a third eyelid, otherwise known as a nictitating membrane. It allows them to minimize the amount of UV radiation entering their eyes, and thus prevent snowblindness.

**EDIT:** By the way, humans used to have a third eyelid as well. And there are vestigial remains of your third eyelid in the corner of your eyes, permanently folded toward the nose. It's called the plica semilunaris, and it's a pretty good example of a useful trait which, unfortunately, was selected out through evolution.