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)

Members of the Al-Baraa bin Malek batallion, part of the Free Syria Army's Al-Fatah brigade, duck to the ground as they pull a man (R) who was shot by a sniper twice in the Bustan al-Basha district of the northern city of Aleppo on October 20, 2012. (Photo by Javier Manzano/AFP Photo)

The car driven by Scott Dixon, of New Zealand, goes over the top of Jay Howard, of England, in the first turn during the running of the Indianapolis 500 auto race at Indianapolis Motor Speedway, Sunday, May 28, 2017, in Indianapolis. (Photo by Marty Seppala/AP Photo)

A student of the Orchestra of Recycled Instruments of Cateura holds an instrument made from recycled material by craftsman Nicolas Gomez, in Cateura, near Asuncion, May 9, 2013. The orchestra is the brainchild of its conductor Favio Chavez, who wanted to help the children of garbage pickers at the local landfill, and the instruments are made from salvaged materials by craftsman Gomez. The orchestra now involves 30 schoolchildren who have toured countries in Latin America, North America and Europe to play music ranging from Beethoven and Mozart to the Beatles and Paraguayan folk songs. (Photo by Jorge Adorno/Reuters)

Muslim children play sack race during Eid-al-Fitr celebration in Taguig city, south of Manila July 18, 2015. (Photo by Romeo Ranoco/Reuters)

An undercover police officer, who had been marching with anti-police demonstrators, aims his gun at protesters after some in the crowd attacked him and his partner in Oakland, California December 10, 2014. Police said more than 100 demonstrators marched in Berkeley, California, which has a history of social activism. Under cloudy skies, turnout was smaller than earlier in the week, when demonstrators in the area threw rocks at police and shut down a major freeway. (Photo by Noah Berger/Reuters)

Australia's Richie “Vas” Vaculik maneuvers past another surfer ducking under the wave during the inaugural Red Bull Cape Fear invitational surfing tournament off the shores of southern Sydney, August 31, 2014. (Photo by Jason Reed/Reuters)

A woman already appears to be regretting her choice of heels for the day out during the Grand National Festival at Aintree Racecourse on April 8, 2017 in Liverpool, England. (Photo by Alamy Stock Photos)