Nictitation, meaning “winking, blinking,” is the noun form of “nictate,” the verb that means “to blink.” For people, blinking is an involuntary process that keeps the eyes lubricated and protects them from any debris. Winking is a purposeful one-eyelid nictitation that serves as a subtle (or not-so-subtle) form of nonverbal communication to demonstrate humor or flirtation. Many animals conduct involuntary nictitation for the same reasons as people, but some species have specialized anatomy called a nictitating membrane. This is a transparent or translucent third eyelid that can be drawn across the eye for protection, while still maintaining sight. Many species of reptiles and birds have full-coverage membranes, but many mammals have evolved to have only a partial membrane. For example, when a dog is sleeping, you might be able to see a partial nictitating membrane across their eye.