Why do zebras have stripes?

No, that's not the opening line of a puerile joke, it's a legitimate avenue of scientific study. Scientists have posited all sorts of possible solutions over the years, from staying cool to a attracting a mate to evading predators, none of which have gained much traction.
Now, though, researches at the University of Bristol and UC Davies California have come up with a reason that makes a lot more sense: the zebra's stripes confuse parasites like horse flies. It turns out that horse flies land much more often on horses than on zebras. The flies aim for the zebras, but then seem to get confused, and do not slow down to land like they do with horses. The stripes seem to confused the low-resolution  visual system of the flies as they come in to land. This is particularly important in Africa, where horse flies carry dangerous diseases like trypanosomiasis. Furthermore, the more prevalent horse flies are in an area, the greater the striping on the local zebras.
In a second study, the researchers even put zebra coats on horses to confirm that the theory works. And yes, it does. So, you can probably expect to see more horses dressed up as zebras in the future.