A female member of Romania's military adjusts her hat before taking part in the national day military parade in Bucharest, Romania, Friday, December 1, 2017. Thousands of Romanian troops staged a military parade to celebrate Romania's national day, but key politicians didn't attend, signaling tensions between the president and the ruling left-wing coalition over plans to revamp the justice system. (Photo by Vadim Ghirda/AP Photo)

Masked Palestinian demonstrators hide behind a metal plate during clashes with Israeli soldiers on March 12, 2018 in the West Bank town of Birzeit, near Ramallah, following a protest by students of the Birzeit University against the arrest of the the head of Palestinian student council by an Israeli undercover commando. (Photo by Abbas Momani/AFP Photo)

A soldier from the South Korean army special forces breaks stone plates with her head during a martial arts demonstration for Memorial Day at Korea War Memorial Museum in Seoul, South Korea, Monday, June 6, 2016.(Photo by Ahn Young-joon/AP Photo)

A woman greets a street artist at the San Fermin festival in Pamplona, northern Spain July 8, 2016. (Photo by Eloy Alonso/Reuters)

Auditioning performers follow resident choreographer Erik Sorensen, center back, at the Sydney Dance Company in a routine during castings in Moulin Rouge's current show “Féerie”, in Sydney, Australia, Thursday, July 28, 2016. The show's artistic team is in Australia to choose new talent to perform with one of the most famous cabarets that has been illuminating Paris since 1889. (Photo by Rob Griffith/AP Photo)

Nigel Lamb from Great Britain, left, leading Juan Velarde of Spain, centre, and Martin Šonka of Czech Republic in the FAI Red Bull air race, Isle of Wight, UK on August 9, 2016. (Photo by Joerg Mitter/Red Bull/Global New/PA Wire)

Brain-on-a-chip. Dazzling in green and magenta this image shows the nerve fibres (in green) produced by neural stem cells (in magenta) as they grow on a synthetic gel. Captured by a technique known as confocal microscopy, the image is part of research shedding light on how tinkering with the environment can affect the way in which nerve fibres grow. (Photo by Collin Edington and Iris Lee/Massachusetts Institute of Technology/Wellcome Images)

US freestyler, gold medalist Mcrae Williams competes during the men' s SlopeStyle (SS) finals at the FIS Snowboard and Freestyle Ski World Championships 2017 in Sierra Nevada on March 19, 2017. (Photo by Albert Gea/Reuters)