Book 31, Question 12
Why do birds have a third eyelid?
A bird has an extra, thin eyelid called a nictitating membrane that slides sideways across the eye.
It is see-through or semi-clear, so the bird can keep watching while it moves. The membrane wipes away dust, spreads moisture, and protects the eye from wind, sand, or splashes. Some diving birds use it like a built-in pair of swimming goggles.