This unit develops concepts of energy and electricity through design, building and testing of toy electric cars. Students first learn to connect motors to batteries, and then to control these circuits with homemade switches. To understand and troubleshoot their circuits, they develop strategies for making diagrams using standard symbols that everyone in the class can agree on. Next they design, make and test simple cars that can be powered by gravity or pushing, and roll freely enough to overcome friction. Subsequently, they add motors to their cars and connect a propeller to the motor. This propeller drives the car in one direction by forcing air in the opposite direction. They learn and apply troubleshooting techniques, and write troubleshooting guides and instruction manuals for fixing and making their cars. Finally, they develop circuits for adding lights to their cars and present their cars and writing to an audience In the extensions, they then explore other drive systems, which include direct- friction- and belt-drive-powered vehicles.