Someday, solar panels may be light and cheap enough that they could be hung on a clothesline, thanks to a synthetic mineral called perovskite. Physicist Sam Stranks explains the solar-powered science and the challenges that stand in its way.

While some world leaders resist global cooperation, city leaders are working across borders to tackle big issues like global warming, immigration and terrorism, says urban expert Robert Muggah.

Here’s a riddle for you: A plane flew for five days and nights, staying airborne for 118 hours. But it didn’t use a single drop of fuel. How? Bertrand Piccard explains.