Even when they sleep, frogs do not close their eyes. Frogs do, however, have eyelids that blink to protect their eyes from dirty residue and preserve moisture. Frogs also have a third eyelid, called the nictitating eyelid, that facilitates a clear view when swimming underwater or on land.
Frogs have a stunning array of eye colors that are attractive to predators. In addition, the backs of a frog's eyes reflect light, so the semitransparent nictitating membrane helps shield the frog from detection by dulling its eye color and diminishing reflection. A frog blinks when it swallows prey, which pushes the eyeballs down. This creates a swelling in the roof of its mouth that helps push the food into the back of its throat.