Gas-masked men of the British Machine Gun Corps with a Vickers machine gun during the first battle of the Somme, 1916. (Photo by General Photographic Agency/Getty Images)

A young Shiite Muslim child participates in a Muharram procession in Ahmadabad, India, Sunday, October 9, 2016. (Photo by Ajit Solanki/AP Photo)

In this photo taken on Friday, July 14, 2017, a student at a paramilitary camp for children calls the rank to attention outside Kiev, Ukraine. As the deadly conflict in eastern Ukraine entered its third year, some parents in Ukraine are anxious to make sure their children are ready to fight it, instead of swimming and playing volleyball. (Photo by Efrem Lukatsky/AP Photo)

A worker lifts up tiles during a bodybuilding contest for tile factory workers as part of celebrating Indonesia Independence day at Jatiwangi village in Majalengka, Indonesia West Java province, August 11, 2017. Indonesia Independence day is on August 17, 2017. (Photo by Reuters/Beawiharta)

A visitor is reflected in a light installation at Mulberry Lights on Regent Street in London, UK on November 16, 2018. Visitors have fun taking selfies and posing with the colourful light installations. Mulberry Lights is a series of illuminating interactive experiences that celebrate how light, colour, shapes and people come together to create a modern British Christmas. (Photo by Imageplotter/Rex Features/Shutterstock)

Sukhoi Su-30SM jet fighters of the Russkiye Vityazi (Russian Knights) aerobatic team perform during a demonstration flight in Krasnoyarsk, Russia October 6, 2018. (Photo by Ilya Naymushin/Reuters)

Hemispheric color differences on Saturn's moon Rhea are apparent in this false-color view from NASA's Cassini spacecraft in this March 2, 2010 file photo. (Photo by Reuters/NASA/JPL/SSI)

A detained protester escapes from a police truck after riot police released teargas to disperse the #OccupyHarambeeAve demonstration in Kenya's capital Nairobi November 25, 2014. Kenyan police used teargas to disperse demonstrators shouting “President, Stop the killings!” outside President Uhuru Kenyatta's offices on Tuesday, in the protest over 28 people killed in a weekend attack claimed by Islamist militants. (Photo by Thomas Mukoya/Reuters)