SKYSCRAPERS typically take a long time to build. The top 20 tallest (currently completed) towers in the world took, on average, 5.3 years to construct. But a company in China called Broad Sustainable Building, with a track record of putting up buildings in a jiffy, is now planning to construct the world’s tallest building—838 metres—in just 90 days. It will be called Sky City One and is to be built in Changsha, capital of Hunan province. This speed is made possible by a novel technique: most of the components are prefabricated on the ground, so the construction process is more like stacking Lego blocks together than building everything on site. The result, if the schedule is adhered to, will be that the building grows by an average of nine metres per day. That is ten times the rate of the next-fastest skyscraper in our chart, the Empire State Building, which was completed in 1931. Quite a tall order, in short.