A cat has upper and lower eyelids that meet when the eye closes, along with a mysterious third eyelid—more properly called the palpebra tertia, also known as the nictitating membrane or “haw.” Tucked in the inner corner of the cat’s eye, the third eyelid is a biological curiosity long thought to be irrelevant, much like the human appendix or wisdom teeth. Veterinary articles written in the early 1900s even described methods for removing the extra eyelid to make it easier to examine the eye. But the third eyelid actually plays an important role in maintaining the eye’s health. In fact, most mammals and birds have a third eyelid—those lacking one, such as humans and some of our fellow primates, are the true oddities.