Sheep gave Henk Verweijmeren his epiphany. The flocks--warming themselves on a stretch of Scottish road--inspired him to create Invisible Heating Systems, which today looks to transform every stretch of asphalt into a solar cell. Cool water pumped through a grid of pipes beneath the road is warmed by the sun and, in turn, warms the surrounding gravel and sand--which stores it, Verweijmeren says, with around 95% efficiency. The hot water in the pipes can be used to melt snow on roads, on runways, or even to fill your water heater. "In Holland," he says, "the government determined that if only 15% of the motor-ways were paved with this, it would produce more energy than all the utilities in that country combined."