Street fighting in Berlin between Government troops and Spartacists, during the Spartacist uprising which followed Germany's defeat in World War I. (Photo by Keystone/Getty Images). 1919

An F/A-18 Hornet assigned to Strike Fighter Squadron One Five One (VFA-151) emerges from a cloud created when it broke the sound barrier in the skies over the Pacific Ocean, July 7, 1999. (Photo by John Gay/US Navy)

A .44 Colt Dragon with a traditional Cowboy's hat. This weapon was originally used in the Mexican War of 1846-8 and it is typical of the type used in the “Wild West” of America. (Photo by Orlando /Three Lions/Getty Images). Circa 1950

A young exhibitor arrives with her kitten on a lead at the National Cat Club show at Crystal Palace, London. (Photo by Fox Photos/Getty Images). 2nd December 1931

Iraqis run for cover as they flee Mosul's Al-Tayaran neighbourhood on February 28, 2017, during an offensive by Iraqi forces to retake the area from Islamic State (IS) group fighters. Hundreds of civilians fled through the desert to escape fighting and privation in Mosul, joining thousands of others who left their homes as conditions worsen in the city's west. (Photo by Ahmad Al-Rubaye/AFP Photo)

Aerobatic aircrafts fly during a performance at the Airshow Zhengzhou 2017 in Zhengzhou, capital of central China's Henan Province on April 27, 2017. The 5-day air show kicked off in Zhengzhou on Thursday, aiming to build a general aviation gathering center for business and communications. (Photo by Li An/Xinhua via ZUMA Wire)

Competitors in the Big Air challenge at the Proryv-2016 festival of extreme sports in Moscow, Russia on March 27, 2016. (Photo by Xinhua/Rex Features/Shutterstock)

A roadside ice-cream vendor waits for customers at a market in New Delhi, India, April 7, 2016. The Indian economy is characterized by the existence of a vast majority of informal or unorganized labor employment. (Photo by Altaf Qadri/AP Photo)