Knowing something like the back of your hand supposedly means that you’re very familiar with it. But it could just as well mean that you think it’s wider and shorter than it actually is. As it turns out, our hands aren’t as well known to us as we might imagine. According to Matthew Longo and Patrick Haggard from University College London, we store a mental model of our hands that helps us to know exactly where our limbs are in space. The trouble is that this model is massively distorted.