Elon Musk's fondness for ambitious, out-of-left-field ideas isn't restricted to electric cars or rockets. During SolarCity's latest earnings call, the entrepreneur revealed that his energy-focused company is developing a "solar roof" -- not a roof with solar panels on it, a roof made out of solar cells. As Musk explains, people install millions of new roofs every year. Why not have them embrace solar power when they look for a replacement? Homeowners with old roofs tend not to install solar panels, since they know they'll have to rip everything off before long.