Riot police walk past burning garbage containers during heavy clashes with demonstrators during a 24-hour strike on March 29, 2012 in Barcelona, Spain. Spanish workers staged a general strike to protest the government's latest labour reforms, which are designed to help Spain lower its deficit within EU limits. (Photo by David Ramos/Getty Images)

Two robotic legged squad support system (LS3) machines by the Defense Advanced Research Project Agency follow a technician during a demonstration at Joint Base Myer-Henderson Hall, Virginia, on September 10, 2012. (U.S. Marine Corps photo by Sgt. Mallory S. VanderSchans via The Atlantic)

“The Lockheed Martin X-33 is an unmanned, sub-scale technology demonstrator suborbital spaceplane developed in the 1990s under the U.S. government-funded Space Launch Initiative program”. – Wikipedia

Photo: An Artist's Rendering Shows How Nasa's X-33 Technology Demonstrator, A Cost-Cutting Fully Reusable Single-Stage-To-Orbit Space Vehicle, Will Look Upon Completion. The 67 Foot-Long, 278,600 Pound, Twin Engine Space Vehicle, Capable Of Reaching Speeds In Excess Of Mach 15, Will Lead To The Construction Of A 127 Foot-Long, 2,186,000 Pound Version With Seven Engines And A 50,000 Pound Payload Capacity Called The “Venturestar”. (Photo By Nasa/Getty Images)

United States Marines perform a fighting demonstration in Times Square as part of Fleet Week festivities May 27, 2011 in New York City. Fleet Week, which has been held in New York since 1984, brings thousands of military members to the city where they engage the public with numerous activities, demonstrations, tours and contests. Fleet Week concludes on Memorial Day with a military flyover to honor those killed while serving in the military. (Photo by Spencer Platt/Getty Images)

A demonstrator wears a mask during a protest inside the Bay Area Rapid Transit (BART) Civic Center station on August 15, 2011 in San Francisco, California. The hacker group “Anonymous” staged a demonstration at a BART station this evening after BART officials turned off cell phne service in its stations last week during a disruptive protest following a fatal shooting of a man by BART police. (Photo by Justin Sullivan/Getty Images)

Anti-nuclear activists demonstrate during a “Say to Goodbye to Nuclear Energy” protest on September 11, 2011 in Kobe, Japan. Japan is marking sixth months since a magnitude 9.0 earthquake struck Japan offshore on March 11 at 2:46pm local time, triggering a tsunami wave of up to ten metres which engulfed large parts of north-eastern Japan and also damaged the Fukushima nuclear plant, causing the worst nuclear crisis in decades. The current number of dead and missing is reportedly estimated to be 22,900. (Photo by Buddhika Weerasinghe/Getty Images)

A protester holds a sign in front of riot policemen during an anti-G20 demonstration through the streets of Nice on November 1, 2011 in Nice, France. Anti-G20 demonstrators are gathered in Nice ahead of the arrival of the world's top economic leaders for the G20 summit in Cannes on November 3rd and 4th. The leaders are expected to debate current issues surrounding the global financial system in the hope of fending off a global recession and finding an answer to the Eurozone crisis. (Photo by Christopher Furlong/Getty Images)

In this photograph taken on February 18, 2016, Pakistani children ride on swings in the predominantly Pashtun Korangi District of Karachi. In a rundown district of Karachi, Rabia balks at a neighbour's proposal to vaccinate her children, demonstrating one of the biggest hurdles to eradicating polio in Pakistan by the end of the year: confused and frightened parents. (Photo by Asif Hassan/AFP Photo)