Pure energy. That's your kid, right? Yeah, us too. Efficiency would be for someone to invent a fun toy that harnesses that indefatigable kid energy (a renewable resource) for fun. Enter the aerodynamic PlasmaCar. We tried one out this weekend ourselves and suffered some light-hearted teasing by onlookers. PlasmaCar operates using the forces of inertia, centrifugal force, gravity, and friction. You add a kid (or adult weighing less than 220 lbs) and a smooth, flat surface and voila! Fun is served. Made of state-of-the-art, high-quality ABS plastics, PlasmaCar has six wheels, a seat, footrests, and a steering wheel. There are no pedals or gears, and it needs no batteries or other power source beyond kid power. So beyond the squeals of delight, it's quiet. Simply rotate the steering wheel continuously back and forth, and PlasmaCar moves forward (up to speeds of 10 km/h). To stop, just cease rotating the steering wheel and put your feet on the ground Fred Flintsone-style to bring it to a full stop. It comes in different colors and is about $70 here and available elsewhere. For those who are interested, the physics behind Plasma Car is explained here.