An eyelid is the thin fold of skin that covers and protects your eye. You have two eyelids for each eye: an upper one that moves down and a lower one that mostly stays in place. When you blink, your upper eyelid quickly sweeps down like a windshield wiper, spreading tears across your eye to help keep it moist and clean.

Your eyelids do crucial work without you even thinking about it. They automatically close when something flies toward your face, protecting your delicate eyeball from dust, insects, or fingers. They blink about 15 to 20 times per minute, which helps keep your eyes from drying out. When you sleep, your eyelids stay closed for hours, giving your eyes darkness and rest.

The inside of your eyelid is very smooth and moist, which is why closing your eyes feels comfortable rather than scratchy. Some people can do unusual things with their eyelids, like flipping them inside out, while others struggle to keep their eyelids open when they're fighting sleep in class.