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)

A reveler, dressed as a Brazilian national army soldier holding a mock weapon with flowers jutting out of it, participates in the annual Gay Pride Parade in Sao Paulo, Brazil, Sunday, June 18, 2017. (Photo by Nelson Antoine/AP Photo)

A woman dressed up as a zombie participates in a zombie walk in Stockholm, Sweden on August 19, 2017. (Photo by Jonathan Nackstrand/AFP Photo)

Fashion lovers are seen gathering outside the shows during Paris Fashion Week, France on October 3, 2017. The over-the-knee boot trend is oh-so real this season thanks to Balenciaga, whose spandex boots have been flying off the shelves. Fashionista’s throughout fashion week battled blisters in order to look on-trend in the footwear. (Photo by Katz Sinding)

Aiden Haas, left, and Lilli Alcala pose after being crowned Mud King and Mud Queen at the 29th annual Mud Day, Tuesday, July 12, 2016 in Westland, Mich. The event, open to children 12 and younger, was at the Hines Park-Nankin Mills Area west of Detroit. Children 12 and younger were able to slosh around in the pit while participating in mud limbo, wheelbarrow races and other free activities. (Photo by Carlos Osorio/AP Photo)

In a handout picture released by the British Ministry of Defence via their Defence News Imagery website on August 22, 2016, Nils Olav the penguin inspects the Guard of Honour formed by His Majesty the King of Norway's Guard on August 22, 2016 at Edinburgh Zoo in Scotland. His Majesty the King of Norway's Guard paid a very special visit to RZSS Edinburgh Zoo to bestow a unique honour upon resident king penguin Sir Nils Olav. Already a knight, the most famous king penguin in the world was given the new title of “Brigadier Sir Nils Olav”. (Photo by Mark Owens/AFP Photo)

In this Monday, October 8, 2018 photo, Colombia's BMX freestyle athlete Sebastian Martinez practices at the Urban Park, at the Youth Olympic Games in Buenos Aires, Argentina. (Photo by Ian Walton/OIS/IOC via AP Photo)

Rwandan refugees cross the Rusumo border to Tanzania from Rwanda carrying their belongings, goats, mattresses and cows, May 30, 1994. The bloodshed that claimed 800,000 Tutsi and moderate Hutu lives began 25 years ago on April 7, 1994, when a plane carrying Rwandan President Juvenal Habyarimana, Burundi President Cyprien Ntaryamira and a French air crew was shot down. (Photo by Jeremiah Kamau/Reuters)