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)

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)

Law enforcement officers clash with protesters, Wednesday, July 20, 2016, in Cleveland, during the third day of the Republican convention. (Photo by John Minchillo/AP Photo)

Indian models showcase creations by designer Shivan & Naresh during the Lakme Fashion Week (LFW) Winter/Festive 2016 in Mumbai on August 27, 2016. (Photo by Sujit Jaiswal/AFP Photo)

Porters carry the carcasses of a shark (L) and a Marlin (R) to the local market in Hamarweyne near the port of Mogadishu on the Indian Ocean coast on November 18, 2014 in the Somalia capital as fishermen bring in their catch. (Photo by Mohamed Abdiwahab/AFP Photo)

Belgium. Rosalie, 9, goes to school in Brussels. “At my school we have separate toilets for girls and boys on every floor. My classroom is on the 3rd floor. We have 22 toilets, which are shared between 230 pupils and 20 adults. The teachers at school let us go to the toilet whenever we need to”. (Photo by Tim Dirven/WSUP/Panos)

Young revelers take part in a parade called "La Calabiuza" on November 1, 2015, on the eve of the Day of the Dead in Tonacatepeque, 20 kms (13 miles) north of San Salvador. During the celebration, the residents of Tonacatepeque, originally an indigenous community, recall the characters from the mythology of Cuscatlan – pre-Columbian west and central regions of El Salvador – and their dead relatives. (Photo by Marvin Recinos/AFP Photo)