A competitor participates in the Tough Mudder challenge near Henley-on-Thames in southern England May 2, 2015. (Photo by Eddie Keogh/Reuters)

A helicopter drops water over a wildfire as a firefighter works to contain it in Benahavis, southern Spain, May 19, 2015. (Photo by Jon Nazca/Reuters)

A general view shows the part of the skyline of Dubai covered in an early morning fog on October 5, 2015. (Photo by Rene Slama/AFP Photo)

A woman sells vegetables near a Metro train station in downtown Lima, Peru December 1, 2015. (Photo by Mariana Bazo/Reuters)

Participants pose for a photographers during the Miss Iraq Pageant in Baghdad, December 19, 2015. (Photo by Ahmed Saad/Reuters)

A clothing shop displays its merchandise in the rebel-controlled city of Idlib, Syria November 7, 2015. (Photo by Ammar Abdullah/Reuters)

A police officer sprinkles colored powder onto a police dog at Nepal's Central Police Dog Training School during a dog worship day as part of the Diwali festival, also known as Tihar Festival, in Kathmandu, Nepal, 10 November 2015. The Tihar festival is the second major festival for Nepalese Hindus and this year is held from 10 November 2015. (Photo by Narendra Shrestha/EPA)

Alexo Carmona, 2, looks at Coco, a two-year-old pony, in downtown Havana, in this March 6, 2015 file photo. Nearly a quarter of a million people follow the Reuters Instagram account – and it's still growing fast. To mark the mid-point of 2015, Reuters has compiled the twenty most-liked pictures so far. This picture was 20th most popular. (Photo by Alexandre Meneghini/Reuters)