A woman takes part in a four-minute flash mob pillow fight at the Old Town Square in Prague, Czech Republic, December 22, 2016. (Photo by David W. Cerny/Reuters)

A clown performs next to the Monument to the Revolution as he takes part in the Latin American Clown Convention in Mexico City, Mexico, October 21, 2015. The convention will be held from October 19 to 22 in Mexico City. (Photo by Henry Romero/Reuters)

A child dressed as a cat takes part in the Children's Halloween day parade at Washington Square Park in the Manhattan borough of New York October 31, 2015. (Photo by Carlo Allegri/Reuters)

A dog dressed in a costume takes part in the Pet's Halloween Day parade at El Olivar Park in San Isidro, Lima, October 31, 2015. (Photo by Mariana Bazo/Reuters)

Palestinian girls take part in a ballet dancing course, run by the Al-Qattan Center for Children, in Gaza City November 25, 2015. (Photo by Suhaib Salem/Reuters)

People take part in the "Bison Race" winter extreme run competition in Zhodino, Belarus, December 5, 2015. (Photo by Vasily Fedosenko/Reuters)

People dressed as Father Frost, the equivalent of Santa Claus, and Snow Maiden take part in the contest “Yolka-fest-2014” (Fir-festival-2014) in Minsk December 12, 2014. (Photo by Vasily Fedosenko/Reuters)

The sun rises over the River Brue on the Somerset Levels as temperatures in parts of the south west drop below freezing, on January, 20, 2015. (Photo by Ben Birchall/PA Wire)