Neuroscientists call this trickery the cutaneous rabbit illusion. At its carrot-y roots, the rascal arises from the inaccurate map your brain has of your arm. On your forearm, sensory neurons can separate stimuli that are 1 centimeter apart. (By comparison, your fingertip has sensitivity resolution of 1 millimeter.) The arm’s imprecise measures aren’t a problem if you’re looking at it, but if you turn away, your brain must rely on assumptions: The object on your wrist didn’t transport to your elbow; it must have run there.