Researchers have installed a molecular engine into a "car" just a few billionths of a metre long. Measuring just 3 by 4 nanometres, around 20,000 of the cars could be parked on the tip of a human hair. Jim Tour and colleagues at Rice University in Houston, US, built a chassis and wheels for a nano-car from organic molecules at the end of 2005. The engineless model could only be powered remotely - using a heated gold surface to stop its wheels sticking and an electromagnetic field to drag it forwards. But the new model should be able to propel...