Presenter notes: Start with a claim they will want to argue with. Say plainly: none of you has ever seen an object in your life. Wait for the protest, then explain. Nothing about that desk ever travels into your eye. What travels into your eye is light, and only light, and everything you think you are looking at is a picture your brain has built from the light that arrived. Then set the trap for later: if that is true, why is a dark room dark? The desk is still there and your eyes still work. Do not answer it yet. Tell them that by the end of the period they will know why they can see a wall, why they can see themselves in a mirror, and why those two are not the same thing at all.