To create a science fair project about wheels reducing friction, the exhibitioner could give live demonstrations of how difficult it is to push identical objects across a surface with and without wheels. Friction is the resistance of a body to motion in contact with another body and is directly proportional to the contact area between two surfaces; wheeled bodies move more smoothly over surfaces because their contact areas are reduced.
A perfectly rigid wheel would be in contact with a surface on a single line, effectively making the contact area between the wheel and the surface zero. Because there are no perfectly rigid wheels, however, the contact area, and consequently the friction between the wheel and the surface, is a reduced, but always nonzero, value. The frictional force that is generated when a body moves against a rough surface is higher than that generated by a body which is moving against a smooth one. To add another layer of depth to a science fair project, this effect could be used to create another exhibit where two identical wheeled bodies are pushed with the same force at the same time across two different surfaces: one smooth and another which is rough. The one moving across the smooth surface decelerates more slowly, and consequently it travels farther than the one moving across the rough surface.
Two general ideas. Test various types of material and measure the amount (or relative amount) of Frichtion. You could test wheels made from different materials: wood, plastic and