An iCub robot built by the Italian Institute of Technology tracks a ball in the Robotville exhibition at the Science Museum on November 29, 2011 in London, England. The Science Museum's Robotville exhibition showcases 20 unique and cutting-edge robots from European research laboratories, it is free to enter and runs from December 1–4, 2011. (Photo by Oli Scarff/Getty Images)

A girl attends Friday prayers in front of an army tank in Tahrir Square in Cairo February 18, 2011. Five years ago thousands of protesters took to the streets demanding the end of the 30-year reign of President Mubarak as Egypt became the second country to join the Arab Spring. After weeks of clashes, strikes and protests across Egypt, Mubarak resigned on February 11, 2011. (Photo by Suhaib Salem/Reuters)

Monsoon thunderstorm in Sierra Vista, Arizona. 9 Sept, 2011. (Photo by Steven Maguire)

Penny Farthing cyclists ride near Tower Bridge on June 9, 2011 in London, England. The riders have cycled from Paris ahead of the IG Markets London Nocturne cycling event on June 11, 2011. (Photo by Peter Macdiarmid/Getty Images)

An anti-government demonstrator argues with a Bolivarian National Police officer during a march toward the headquarters of the national electoral body, CNE, in Caracas, Venezuela, Wednesday, May 18, 2016. Opposition protesters were blocked from reaching the CNE as they demand the government allow it to pursue a recall referendum against Venezuela's President Nicolas Maduro. (Photo by Fernando Llano/AP Photo)

Demonstrators clash with members of the Bolivarian National Police as protests continue in Caracas, Venezuela, 20 April 2017. The Bolivarian National Guard, using tear gas, tried to disperse protests against the government of Venezuelan President Nicolas Maduro. (Photo by Miguel Gutierrez/EPA)

Dancers of the Czech National Ballet warm up backstage during a performance of “The Nutcracker – A Christmas Carol” at the National Theatre in Prague, Czech Republic, late 16 December 2017. Peter I. Tchaikovsky's “The Nutcracker”, one of his most famous compositions and seasonal ballets, premiered in St. Petersburg, Russia, on 18 December 1872, celebrates its 125th anniversary this year. (Photo by Martin Divisek/EPA/EFE)