It’s the job of our eyelids to protect our eyes. The eyelids slam shut at a loud noise, bright light, or quick movement near our face. A closed eye is waterproof – and airtight. And our eyes need to stay moist. When we blink – about once every five seconds- the lids mop our eyeballs with tears.

The tears lubricate the eye and wash away specks of dust. Tears also contain an antibacterial agent that helps defend our eyes from airborne bacteria. Used tears drain out through the nasal cavity – that’s why you might sniff a little when you cry.

Most mammals have eyelids similar to ours – but other types of animals don’t need eyelids. For example, fish don’t have true eyelids – they live in water that keeps their eyes moist. Fish can’t close their eyes – and neither can snakes. Some lizards have two eyelids – a thin, clear one to protect the open eye – and another pigmented eyelid to close the eye.