“The Challenge!” Young boy relishes in a game of tug of war before school. Photo location: Lake Titicaca, Peru. (Photo and caption by Paul Heyman/National Geographic Photo Contest)

Labourers work at the installation site of Liquefied Petroleum Gas (LPG) pipeline on May Day or Labour Day in Chandigarh, India, May 1, 2015. (Photo by Ajay Verma/Reuters)

In this photo taken on Thursday, July 23, 2015 migrants enter a train to Serbia at the railway station in the southern Macedonian town of Gevgelija. The country has become a major transit route for thousands of Middle Eastern and African refugees and migrants who cross over from Greece and then continue into Serbia. (Photo by Boris Grdanoski/AP Photo)

Ratana Das, 40, a female vendor, carries sacks full of clothes at a second-hand clothing market early morning in Kolkata, India, March 10, 2016. Das said she was born and brought up in Kolkata and feels proud being a resident of the city that gives her he opportunity to be financially independent. (Photo by Rupak De Chowdhuri/Reuters)

A migrant labourer brushes his teeth in front of a scrapped helicopter model in Mumbai, India, April 13, 2016. (Photo by Danish Siddiqui/Reuters)

A military police officer aims his gun to a demonstrator during a protest in Brasilia, Brazil, Tuesday, April 7, 2015. The officer had picked up a bottle that was thrown towards them but did not fire his gun and backed away. Thousands of workers have staged rallies in 12 cities across Brazil to protest against a proposed law that would allow companies to outsource their labor force. (Photo by Eraldo Peres/AP Photo)

The Northern Lights glow green in a spectacular light show above a field of trees frozen solid with snow. (Photo by Jaak Sarv/Solent News & Photo Agency)

Participants take pictures of themselves as others run past the Tiananmen gate during the Beijing Marathon in Beijing, China, September 17, 2016. (Photo by Damir Sagolj/Reuters)