Walking toy (also called "ramp walkers"—available at specialty toy stores, usually at a cost of $2–$10) or any object that can slide on a table and served as an improvised substitute
Approximately two feet of string
Several paper clips and/or binder clips (mini or micro)
No assembly needed if you are using a ramp walker toy.
If you are using an object that slides, either tie the string directly to the object if feasible or place the rubber band around the object and tie the string to the rubber band.
To Do and Notice
If using a ramp walker toy, hang the mass over the side of a table or desk and let the toy walk toward the edge.
If improvising with a sliding object, attach sufficient mass (paper clips and/or binder clips, or anything that works) to the string so that when the object is about a foot from the edge of the table with the mass hanging over the edge, a slight push on the object will start it sliding toward the edge.
What’s Going On?
Why does the toy walk? For the toy to move, you must apply a force that is at least as great as the frictional force trying to stop it. The weight of the mass pulls along the string and provides the force that results in the toy's motion. The string pulls diagonally, though, and only the horizontal component of the force makes the toy move forward (click to enlarge the diagram below).
As the toy gets closer to the edge, the angle of the pull changes. The component of force pulling forward gets relatively shorter, and the component pulling down gets relatively longer. At the edge of the table, there is no component of force pulling the toy forward, so it stops!