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 general view of the sunrise over Hawes, Wensleydale, UK on November 24, 2014. (Photo by Neil Squires/PA Wire)

Female female male threesome. My bit on the side is my best male friend. (Photo by Laoshi/Getty Images)

The Spanking. (Photo by Vika Valter/Getty Images)

Full length portrait of female skier, Sierra Nevada Mountains in California, USA. (Photo by Anatoliy Gleb/Getty Images)

A man salts a downtown sidewalk in Washington, Thursday, March 5, 2015, as snow begins to fall. The U.S. federal government said its offices in the Washington area will be closed Thursday because of a new round of winter weather expected in the region. The Office of Personnel Management said non-emergency personnel in and around Washington were granted excused absences for the day. Emergency employees and telework-ready employees were expected to work. (AP Photo/Andrew Harnik)

Tango instructor Fernando Waisberg (R) and Isabella Waisberg (L) pose for a photograph during a Tango lecture session in Taipei, Taiwan, 14 April 2019 (issued 18 April 2019). Taiwan has developed its own Tango, with a strong Japanese influence; the accompanying music features lyrics in Taiwanese and Mandarin, the pace slower and simpler than the original. Taiwanese Tango is now a popular phenomenon among people over 40, and is also a subject little studied inside and outside of the island. (Photo by Ritchie B. Tongo/EPA/EFE)

Women in lingerie touching each other. (Photo by BDLM/Getty Images)