Ducks have three eyelids.
(Actually this isn’t particularly amazing, because all birds have three eyelids, and so do dogs and cats. In ducks the upper and lower eyelids have small bristle feathers that resemble eyelashes. They only close their eyelids during sleep, and use the third eyelid alone for blinking.
The third eyelid, the nictitating membrane, lies beneath the eyelids on the side of eye closest to the nostril. It darts across the eye about 30-35 times per minute in the domestic fowl, and also moves across the eye if an object approaches the eye suddenly or if something touches the head. The third eyelid becomes scooplike and sweeps excess fluid in to the corner of the eye where it drains. In most birds, the nictitating membrane is transparent, so vision is not impaired when the eyelid blinks, which is important since so many birds are prey animals. It helps to be able to see when blinking! It is suspected that some birds may fly with the third eyelid covering the cornea of the eye, which prevents it from drying out during flight, acting like birdy goggles.
I added this fact because of a complaint by a reader who was looking for duck facts and ended up on my duck-billed platypus fact. Just goes to show, you can please some of the people all of the time, and all of the people some of the time, but you can’t please all of the people, all of the time.)