Soviet Army soldiers sit on their tanks in front of the Czechoslovak Radio station building in central Prague during the first day of Soviet-led invasion to then Czechoslovakia on August 21, 1968. (Photo by Libor Hajsky/Reuters)

“Himalayan Dog”. This dog just appeared out of nowhere and followed us for an entire week during our trekking trip in the Himalayan outback. He always used to sleep in front of our tent and guarded us in the nights. When I decided to get up at 4 a.m. to climb the next 5000 m peak for sunrise he accompanied me as well. On the top he was sitting for the entire 30 minutes on this place looking straight into the countryside. Photo location: Ladakh, India. (Photo and caption by Sebastian Wahlhuetter/National Geographic Photo Contest)

Tourists take a “selfie” as demonstrators burn a trash container during a May Day rally in Barcelona, Spain, on May 1, 2014. Tens of thousands of workers marked May Day in European cities with a mix of anger and gloom over austerity measures imposed by leaders trying to contain the eurozone's intractable debt crisis. (Photo by Manu Fernandez/Associated Press)

Two men fight in San Jose, Costa Rica, during the Women's Day on March 8, 2014. (Photo by Jeffrey Arguedas/EFE/SIPA Press)

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)

A female goalkeeper saves the ball at the Mathare Environmental Conservation Youth Center during a soccer match to celebrate the International Women's day in Nairobi, Kenya, March 8, 2017. (Photo by Thomas Mukoya/Reuters)

Mermaid Hales Parcells geets children as she performs at the Virginia Aquarium in Virginia Beach, Va., Monday, April 3, 2017. The Aquarium presents a mermaid show one night each week during the month of April. (Photo by Steve Helber/AP Photo)

The Boeoegg, a giant symbolic snowman made of wadding and and filled with firecrackers burns on top of a bonfire in Sechselaeuten square on April 24, 2017 in Zurich. When the bells of the Saint Peter's church chime six o'clock, the bonfire under the snowman is set in fire and the guildsmen wearing historical costumes ride around the bonfire. The faster the head of the snowman explods the warmer and more beautiful the summer will be. (Photo by Michael Buholzer/AFP Photo)