How Does a Frog Close Its Eyes?

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.