You show a wooden cube on a piece of string and let it drop down by gravity. You then state that you can make this cube obey you and defy the laws of gravity. You can now make the cube stop at any point of its free fall. Can be handed out for inspection but no one else can do what you can do. You can repeat it any number of times.
This is a well-made obedient cube. Made out of wood with the four suites of the playing cards stuck on the four faces of the cube. This obedient cube is lacquered to a high glossy finish.