Students from St Andrews University indulge in a tradition of covering themselves with foam to honour the “academic family” on the lower college lawn on October 23, 2017, in St Andrews, Scotland. (Photo by Jeff J. Mitchell/Getty Images)

Katy Perry poses backstage during rehearsals for the 2017 MTV Video Music Awards at The Forum on August 25, 2017 in Inglewood, California. (Photo by John Shearer/Getty Images for MTV)

Participants run through the streets of the Back Bay during the 16th annual Santa Speedo Run in Boston, Massachusetts, December 12, 2015. (Photo by Brian Snyder/Reuters)

Labourers load a three-wheeled vehicle onto the back of a van at the Mercato market in Addis Ababa October 9, 2015. Addis Ababa's “Mercato” – Italian for “market” – is reputedly the biggest open-air market in Africa, lying in the west of the capital. (Photo by Tiksa Negeri/Reuters)

An Indian rag picker collects plastic bags at an industrial area on the outskirts of Jammu, India, Tuesday, June 5, 2018. The U.N. says government bans on plastic can be effective in cutting back on waste but poor planning and follow-through have left many such bans ineffective. (Photo by Channi Anand/AP Photo)

An alligator named Muja is seen in its enclosure in Belgrade's Zoo, Serbia, August 14, 2018. Muja is officially the oldest American alligator in the world living in captivity. He was brought to Belgrade from Germany in 1937, a year after the opening of the Zoo. Muja survived three bombings of Belgrade, the Second World War and all hardships the Zoo went through. (Photo by Marko Djurica/Reuters)

Teeming with images of spectacular underwater scenes from around the world, Call of the Blue is the culmination of a five-year project by the photographer and ocean conservationist Philip Hamilton. This groundbreaking book includes contributions from acclaimed scientists and ocean “guardians”, who reveal what drove them to answer the call of the blue. (Photo by Philip Hamilton/The Guardian)

British sprinter Dina Asher-Smith arrives at The Fashion Awards 2018 In Partnership With Swarovski at Royal Albert Hall on December 10, 2018 in London, England. (Photo by Mike Marsland/BFC/Mike Marsland/Getty Images)