Russian soldiers write home while serving on the Eastern Front during World War I, circa 1915. (Photo by Hulton Archive/Getty Images)

Party-goers partake in a festival better known as Holi One, during International Workers Day in Johannesburg, South Africa, 01 May 2015. The colour festival promotes togetherness as well as colourness in everday life. May Day is observed worldwide every year on 01 May. (Photo by Kevin Sutherland/EPA)

A gibbon drinks from a soft drink bottle, given by a tourist, at a zoo in Kunming, Yunnan province, June 18, 2015. (Photo by Wong Campion/Reuters)

Competitors take part in the 8th annual World Gravy Wrestling Championships at the Rose n Bowl Pub in Bacup, north west England on August 31, 2015. Contestants must participate in fancy dress and wrestle in a pool of Lancashire Gravy for 2 minutes whilst being scored for a variety of wrestling moves. (Photo by Oli Scarff/AFP Photo)

Turkana tribesmen walk with guns to protect their cattle from rival tribes near Baragoy, Kenya August 4, 2017. (Photo by Goran Tomasevic/Reuters)

Balinese man watches Mount Agung volcano almost covered with clouds as he stands at a temple in Karangasem, Bali, Indonesia, Tuesday, September 26, 2017. An increasing frequency of tremors from the volcano indicates magma is continuing to move toward the surface and an eruption is possible, a disaster agency official said Tuesday. Tourists are cutting short their stay to the island, where an eruption would force the airport to close and strand thousands. (Photo by Firdia Lisnawati/AP Photo)

Spectators pass through security screening ahead of the New Year's Eve celebration in Times Square in New York, on Sunday, December 31, 2017. (Photo by Peter Morgan/AP Photo)

In this January 3, 2018 photo several female Gelada baboons, also known as bleeding-heart baboons, cuddle with their youngs in order to keep warm at the Wilhelma zoo in Stuttgart, Germany. (Photo by Sebastian Gollnow/DPA via AP Photo)