Now researchers at MIT have taken new tack. Instead of adding pieces, they first pile the bits together, then shed the parts they don't need to leave the desired shape. Their 'smart sand' is made up of individual grains that are really one centimetre square cubes with microprocessors inside and switchable magnets on four of their six faces which can talk to each other electronically and sense their neighbours.

The team demonstrated how they work in two dimensions by placing a 2-D 'footstool' inside a grouping of blocks. The magnets are turned on, and the modules check what is next to them on all four sides of the plane. Then an algorithm calculates how to create the same pattern in an adjacent region of block, and switches off magnets on the faces of the module corresponding to those next to the footstool. This forms a copy of the original object. Computer models show the same approach can work in three dimensions.