KS23: Collect full-face pictures of celebrities and make two copies, one of which is on overhead transparency. Then cut each one in half, lengthways. Stick two right-side faces and left-side faces together. This reveals a different face from the original and could lead into a discussion of bilateral symmetry in humans and other animals, and radial symmetry in marine animals such as jelly fish.
KS34: We can be fooled in the way we perceive objects and exercises, and using optical illusions show how this happens. Junior texts, such as Spotlight Science (Nelson Thornes, pound;8.77) have simple, photocopiable illusions. Optical Illusions by Bruno Ernst (Taschen, out of print, but available through libraries), has more complex illusions, including photos of a concave face by Sandro Del-Prete, which the brain interprets as convex. Ask Jeeves for Kids (www.ajkids.com) contains explanations of how some illusions work.
Memory games are also useful, especially those that can be used to demonstrate successful learning and revision techniques. Try You and Your Mind: Students' Book (Longman, out of print, but available through libraries) for some simple examples.