Sensational - the try 'em at home guide to illusions

The Touch Illusion

If you raid a DIY toolbox you can try the sandpaper Touch Illusion. Carefully rub one hand on fine sandpaper, the other on coarse sandpaper. Now take both hands and rub some medium sandpaper. It feels different to each hand. Why? Because the hand rubbing the fine sandpaper first adapts to feeling fine roughness, whereas the hand on the course sandpaper adapts to lots of roughness. So like the temperature illusion when your two hands have had their touch sensors adapted to different roughness, the medium paper will feel different to each. Try to predict, from the Temperature Illusion explanation, which hand will feel the medium paper as more rough, then try the experiment and see if you are right.