This is an optical illusion effect that is a real show stopper. It comes with a hand held motor that is reversible.
The group is asked to stare at the very center while it is in motion. In order to work best, have them stare for 30 seconds. In the last five seconds you count down from 5 to one. When you get to one, have them look at your face. When they do this, your head will expand or contract depending on what direction the wheel is moving. When you reverse the direction of the spin, the opposite reaction will occur. It works with any number of people. It is an excellent metaphor for the importance of focus and can be used for science when studying the eye and its muscles.