Need a building? Just add water.That's the plan that a pair of engineers in London have come up with. "Building in a bag." To erect the structure, all you have to do is add water to the cement-impregnated fabric and inflate it with air. Twelve hours later you have an instant Quonset hut. The bag weighs 230 kilograms (~500 pounds) before adding the water, and inflates into a shelter with 16 square meters (172 square feet) of floor space. Cost is estimated at $2,100, while an equivalent-size portable building costs about $7,700. A similar-size tent costs about $1,150. The volume of the bag controls the water-to-cement ratio, eliminating the need for water measurement. You literally just add water. One wag has dubbed it "The Chia Shelter."