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Twin tornadoes can mow down small towns in minutes, and although unusual they can develop in several ways. Most tornadoes, either single or double, are associated with supercell thunderstorms, which have a large, vertical column of rotating air. When wind blows in different directions and at varying speeds, supercells tend to form tornadoes. This is where it gets tricky. Jane J. Lee has done

f you took all the world's population and dumped them into the Grand Canyon they wouldn't fill a fraction of it. In fact the 7.2 billion people would only form a comparatively tiny pile - as shown in this fascinating mock-up image. The graphic was put together by respected YouTube presenter Michael Stevens on his Vsauce channel, which juxtaposes the population of the world alongside the famous d