Breaking a mirror means seven years bad luck. So does spilling salt or meeting a black cat. We've all heard such silly-sounding superstitions. Of course why anybody would believe that stepping on a crack could break your mother's back is a mystery. But according to an article in the Royal Society journal Biological Sciences, superstitious behaviors are a natural product of evolution. Imagine an animal living in an environment where, over the course of a day, he might hear some rustling in the leaves or maybe in the grass. Now, movements in the grass could signal a predator attack, whereas the breeze in the trees is probably just the wind. Still, the animal has a choice: he can ignore all this rustling and go about his business, or he can run and hide. The most logical response would be to hide only when he hears the grass move. But what if it's hard to tell whether the noise came from the grass or the trees? "l could've sworn that was the trees" could be his final thought. So the animal learns to bolt at the sound of the breeze, because it could foretell certain doom. That better-safe-than-sorry attitude is essentially a superstition. One that that cautious critter will likely pass on to his young. Knock on wood.