Here is an easy and inexpensive way to demonstrate how gears work. You might want to make the gears in various sizes to demonstrate how the gears on a bicycle make it easier to pedal uphill. Plus, it's a great way to recycle old cardboard boxes!
Choose a piece of corrugated cardboard. It should be at least 12 inches on one of the sides that is perpendicular to the corrugates. In others words, along that 12-inch side you should see the wavy, undulating lines of the corrugates. You don't have to use one large piece of cardboard. You can use miscellaneous small pieces.
Use the compass to draw circles on one part of the corrugated cardboard (suggested sizes to start out with: 1 1/2 inch diameter, 2 1/4 diameter and 3 1/8 diameter). You can cut as many circles as you want to and you can experiment with the sizes the circles. (Hint: Make sure to push the compass pin point into the cardboard enough so that it leaves a visible mark. You will need to know where the centre of the circle is when you go to assemble the gears.)
Use the scissors to cut out the circles. (Hint: You may find it easier to cut the circles if you cut out a rough shape around them first, then refine and trim the shape.)
Use the sharp craft knife or razor blade to cut a thin strip off one end of the cardboard. You will be cutting across the wavy corrugates. The strip should be at least 3/16 inch thick but no more than 1/4 inch thick. After you cut the strip, peel the brown paper off one of the sides.
Wrap the thin strip around one of the circles and snip it off so that the ends meet and are flush. Use white glue to glue the strip around the circle.
Use a piece of tape to hold the ends of the strip around the circles while the glue dries.
Do these same steps to all your circles so that you have a set of gears.
Use push pins to attach the gears to a piece of cardboard. Use the hole left in the centre by the compass as your guide for where to insert the pin. The gears should slide easily on the push pin. If they do not, move the pin around in the hole a bit to enlarge the hole just slightly. You can add a handle to any of the gears by inserting a push pin near the outer edge. As you turn the handle, all the gears should move. If they do not, adjust the gears a bit. The corrugates should mesh together so that the gears turn easily.