http://wired.com/news/technology/0,1282,66872,00.html
A pair of engineers in London have come up with a "building in a bag" -- a sack of cement-impregnated fabric. To erect the structure, all you have to do is add water to the bag and inflate it with air. Twelve hours later the Nissen-shaped shelter is dried out and ready for use.
Now, that's clever.