Wedges of an orange generate enough current and electrical juice – 3.5 volts – to power an LED. The fruit’s citric acid helps electrons flow from galvanized nails to copper wire in this 14-hour exposure. This image was published in September’s Visions of Earth, a trio of photos that appear in each issue of National Geographic. (Photo by Caleb Charland/National Geographic)

A young woman wearing no trousers hits the ski slopes at a ski resort in Xuzhou, in east China's Jiangsu province on January 13, 2015 in a promotional effort to equal “No Pants Subway Ride”, which was marked on January 11 this year. The promotional “No Pants Subway Ride” stunt on January 11 was held as commuters in some 60 cities around the world braved public transportation in their undies, which has gone global since its first staging by US group Improv Everywhere in New York in 2002. (Photo by AFP Photo)

Refugees and migrants try to find a place on an already full train headed for Serbia, near the city of Gevgelija, The Former Yugoslav Republic of Macedonia, on February 24, 2016. Refugees and migrants from Iraq and Syria continue to pass through Macedonia on their way to the European Union. (Photo by Georgi Licovski/EPA)

Children exercise before a wrestling practice session at an old Basque ball game gymnasium in downtown Havana, October 30, 2014. (Photo by Alexandre Meneghini/Reuters)

Selfie picture shows Swiss pioneer Bertrand Piccard during the last leg of the round the world trip with Solar Impulse 2 over the Arab peninsula July 25, 2016. (Photo by Jean Revillard/Bertrand Piccard/Reuters/SI2)

A couple watches the sunset over a sea of fog covering the Geneva Lake from Chardonne, Switzerland, 28 December 2015. (Photo by Jean-Christophe Bott/EPA)

Macaque monkeys crowd together in their cage at a monkey farm on February 3, 2016 in Xinye county, Henan province, China. The area boasts a centuries-long and lucrative history of raising and training monkeys for performance. In Xinye, villagers are seeing an increase in business with the lunar calendar's “Year of the Monkey”. Farmers say most of the monkeys are bred and raised for domestic zoos, circuses, and performing groups, but add that some are also sold for medical research in China and the United States. Despite the popularity of the tradition, critics contend the training methods and conditions constitute animal cruelty. (Photo by Kevin Frayer/Getty Images)