Many people wear bifocal eyeglasses, which have two different kinds of glass in each eye: the lower half for reading and the upper half for looking at things farther away. But there’s a tiny tropical fish, the anableps, that has eyes which work just like bifocal eyeglasses.
The anableps spends a lot of its time swimming near the surface of the water looking for insects to eat. The upper half of the anableps’ eyes are focused for short vision, so that the fish can see insects on the surface.
But the lower half of the anableps’ eyes are focused for underwater vision, so that the fish can watch out for other sea creatures that like to dine on the anableps. This bifocal vision permits the anableps to “keep its eyes on” two different things at the same time!