<http://schema.org/about>
"Inventors Martin Riddiford and Jim Reeves realized they could eliminate the need for a battery if they relied on the power of gravity."^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.mnn.com/leaderboard/stories/what-is-gravitylight-a-5-battery-free-lamp-for-the-poor>