What is Proprioception?
Your sense of proprioception is your brain's ability to sense where your limbs are. Nothing strange about that, right? This is how you can put a sandwich in your mouth while your eyes are focused on the TV. Your brain knows where your hand is in relation to your face, thanks to proprioception. Even though your brain carries around a detailed awareness of exactly where your body parts are at all times, when it's handed a contradictory stimulus, essentially it says, "Oh, well. Guess I've been wrong about the length of your nose all these years."
The best example we've found so far is "the Pinocchio illusion." Scientists have found they can have the subject touch the tip of their nose with their finger, and have their bicep or triceps electrically stimulated at the same time. Your brain "feels" your arm muscle extending, but also feels that you're maintaining contact with the tip of your nose, and leaps to the immediate, yet fully sober, conclusion that your nose has suddenly grown to be about three feet long.