The technical term for what you do when your eyelids close is nictate, or alternately, nictitate. Whether you're blinking in the sunshine or winking at your friend after giving the substitute math teacher a hard time, you nictate. Almost every single animal has the ability to nictate, and even those without true eyelids have a protective membrane that occasionally covers their eyeballs. The Latin root is nictare, "to blink."