Imagine the scene. You spot someone in the distance - but you're not sure if it's another person, or your own reflection. So what do you do? You move. And see if the 'reflection' moves in the same way. Simples!
So simples, in fact, that even cats do it. At least, Sox - the cat above - did it.
Click play to watch our feline friend looking very flummoxed indeed. Although 'tis probably better to be 'flummoxed' than 'so fighty you end up falling over'...
(Via Tastefully Offensive)