A competitor gestures as he crawls beneath barbed wire during the Tough Guy event in Perton, central England February 1, 2015. The annual event to raise cash for charity challenges thousands of international competitors in a cross country run followed by an assault course consisting of obstacles including water, fire and tunnels. (Photo by Phil Noble/Reuters)

A woman wears roller skates with lights on the bottom at Rich City Skate in Richton Park, Illinois, January 12, 2015. A handful of decades-old skating venues put Chicago at the center of a vibrant African-American subculture of urban roller skate dancing that stretches from Atlanta to Detroit and from Los Angeles to New York. While mainstream roller skating has been on a long decline, a new generation of skaters travel a circuit of rinks around the country to compete and show off their moves. (Photo by Jim Young/Reuters)

A boy rides on a snow wagon towed by a robot dressed as a Teddy bear, which only moves forward by moving its legs, during the Ice and snow carnival at Taoranting park in Beijing February 9, 2015. (Photo by Kim Kyung-Hoon/Reuters)

A Polish Air Force MIG-29 fighter (R) and an Italian Air Force Eurofighter Typhoon fighter patrol over the Baltics during a NATO air policing mission from Zokniai air base near Siauliai, Lithuania February 10, 2015. (Photo by Ints Kalnins/Reuters)

A woman dressed as a cleaning lady laughs during “Weiberfastnacht” (Women's Carnival) in Cologne February 12, 2015. Women's Carnival marks the start of a week of street festivals, the highlight of the event being “Rosenmontag” (Rose Monday), the final day of the carnival where mass processions are held. (Photo by Ina Fassbender/Reuters)

A garbage collector, with his horse and cart, prepares to unload rubbish at the municipal dump in Nezahualcoyotl, on the outskirts of Mexico City, February 18, 2015. Hundreds of horse or donkey-drawn carts will disappear from the streets of a municipality in the state of Mexico, located on the outskirts of Mexico City, and will be replaced by motorized vehicles, local authorities said. (Photo by Henry Romero/Reuters)

In this January 27, 2015 photo, penguins walk on the shore of Bahia Almirantazgo in Antarctica. Antarctica “is big and it's changing and it affects the rest of the planet and we can't afford to ignore what's going on down there”, said David Vaughan, science director of the British Antarctic Survey. (Photo by Natacha Pisarenko/AP Photo)

Two F-15E Strike Eagles wait to receive fuel from a KC-135R Stratotanker during exercise Red Flag 15-1 on their way to Nellis Air Force Base, Nev., Jauary 23, 2015. The F-15s are assigned to the 4th Fighter Wing. U.S. (Photo by Airman 1st Class Aaron J. Jenne/Reuters)