The increasingly popular Toyota Prius, a gasoline-electric hybrid, is capable of about 50 miles per gallon. But how about 100 mpg instead? And seven miles of all-electric driving? That's what's possible with more batteries and the ability to recharge them via a standard household outlet. The idea of plug-in hybrids has been around for years, but now they are finally within the near future. Significant hurdles remain, but nearly every automaker has plug-in hybrid technology under development. Toyota, which made hybrids a household term with the Prius, has been cautiously developing a plug-in version of the Prius. Several are now on the road for testing and further development. Read this article to understand how plug-in hybrids work, the challenges of their development, and hear a firsthand account of driving a prototype plug-in Prius.