This simple science experiment will give pupils an opportunity to investigate a magic trick and work out the science behind it.
Before the lesson, take a can with a lid. A coffee tin would work very well. Make two small holes in the base, close to the edge, and on opposite sides. Now make two similar holes in the lid, also close to the edge and on opposite sides, but at right angles to the ones in the base.
Take an elastic band and thread it through the holes in the lid so the ends are on the inside. Likewise, thread another elastic band through the holes in the base, ensuring both ends are inside the jar. Connect all four ends together with a paperclip. This can be quite tricky. Attach a small weight (for example a small nut or bolt) to the paperclip. Replace the lid and tape it down. Roll the tin away from you on a flat surface. It should roll away and then return.
Ask your pupils to work out why the tin behaves in this strange way. Give them an opportunity to explore the tin's behaviour and allow them to test it in different circumstances. What happens if the tin rolls down a slope? How does the tin behave if you roll it quickly or slowly? Shake it gently and try to guess what is inside. What happens if you turn the tin in your hands, and then place it on a flat surface? Once everyone has had a guess, remove the lid and show pupils what is inside. As the tin rolls, the weight remains hanging downwards, causing the elastic bands to twist and store energy. Once the tin stops rolling the elastic bands begin to untwist, the energy is released and the tin rolls backwards. You can make a version in a clear container to see how it works.
Sophie Duncan is programme manager for science at the BBC www.bbc.co.ukscience