A mine detection rat is given banana as a reward after successfully identifying an inactive mine on July 2, 2015 in Siem Reap, Cambodia. The Cambodian Mine Action Center (CMAC) working with the Belgian NGO APOPO has recently begun testing the feasability of using large mine detection rats from Tanzania to help clear fields of mines and unexploded ordnance in one of the most bombed and mined countries in the world. (Photo by Taylor Weidman/Getty Images)

Young gymnasts practice pull-up on a bar at a gymnasium of a sports school in Jiaxing, Zhejiang province April 11, 2015. (Photo by William Hong/Reuters)

Workers hold an 80 Kg Paiche (Arapaima gigas) to return it to a pool at a breeding farm in Nueva Loja, Ecuador, March 25, 2016. (Photo by Guillermo Granja/Reuters)

People play on a beach along the Arabian Sea on a hot summer day in Mumbai, India, April 29, 2016. (Photo by Danish Siddiqui/Reuters)

Hospitality staff members laugh as they stand on Tiananmen Square before the closing ceremony of China's 19th Party Congress at the Great Hall of the People in Beijing, Tuesday, October 24, 2017. The ruling Communist Party on Tuesday formally lifted Xi Jinping's status to China's most powerful ruler in decades, setting the stage for the authoritarian leader to tighten his grip over the country while pursuing an increasingly muscular foreign policy and military expansion. (Photo by Mark Schiefelbein/AP Photo)

People gather at the site of a train derailment near Beni Suef, Egypt, Thursday, February 11, 2016 that injured dozens of people were injured as it was traveling north toward Cairo. Railroad accidents due to negligence are common in Egypt. Egyptians have long complained that the government has failed to deal with the country's chronic transport problems. (Photo by Samer Abdallah/AP Photo)

In this photo taken Tuesday, June 23, 2015, a Ukrainian serviceman from the Kiev-2 volunteer battalion fires an weapon at a frontline in the village of Krymske, east Ukraine. Few places along the front line in east Ukraine see regular fighting as bitter as the village of Krymske. For now, Krymske is in the hands of Ukrainian government forces and the volunteer battalions that fight alongside them. (Photo by Evgeniy Maloletka/AP Photo)

A handout photograph provided by Brian Kubicki of Costa Rican Amphibian Research Center on 26 April 2016 shows a “Crystal frog”, Hyalinobatrachium dianae (H. diane). This frog was discovered by US biologist Brian Kubicki and Costa Ricans Stanley Salazar and Robert Puschendorf in a rainy forest of Costa Rican caribbean after 40 years without notice of any new example of this kind. (Photo by Brian Kubicki/EPA/Costa Rican Amphibian Research Center)