A migrant boy sits on railway tracks at a train station in Tovarnik, Croatia, September 20, 2015. (Photo by Antonio Bronic/Reuters)

A working team appointed by the Archaeological Superintendence of Pompeii performs a Cat scan (Computerized axial tomography) one out of thirty casts of victims of the eruption of Vesuvius in 79 AD in Pompeii, in Napoli, Italy, 29 September 2015. The project which aims to trace the habits of life, employment and social class of the victims, involves a working team of archeologists, anthropologists, radiologists, dentists and engineers to obtain results from the scan. (Photo by Cesare Abbate/EPA)

Following a large air raid by German bombers, local people, coming out of their air raid shelters found a bus standing on end in a large crater, in Balham, London, October 15, 1940. (Photo by AP Photo)

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 commuter gestures to Boris Johnson as he rides across Vauxhall Bridge in London, England on November 19, 2015. The London mayor was opening a cycle superhighway. (Photo by John Stillwell/PA Wire)

In this photo taken Sunday, March 12, 2017, a girl Abuk walks home with her brothers and friends after collecting clean water from a water point four kilometers away from her home, in Aweil, in South Sudan. As World Water Day approaches on March 22, more than 5 million people in South Sudan, do not have access to safe, clean water, compounding the problems of famine and civil war, according to the UNICEF. (Photo by Mackenzie Knowles-Coursin/UNICEF via AP Photo)

A model presents a creation by Chinese designer Chen Wen at China Fashion Week in Beijing, China March 29, 2017. (Photo by Jason Lee/Reuters)

A woman smokes marijuana during the annual 4/20 marijuana rally on Parliament Hill in Ottawa, Ontario, Canada, April 20, 2017. (Photo by Chris Wattie/Reuters)