Phantom limbs or phantom pain is a phenomenon where receptors in the brain pick up pain or movement from a nonexistent limb. For example, if you put your hand where someone's leg would be, they would be uncomfortable, as you'd be putting your hand through their phantom limb. Despite it not being their, your brain may still feel like it is. This is also seen in animals, as a cat or dog with an amputated back leg will try and scratch their ears or sides with a limb that isn't there.
