Scientists at Argonne National Laboratory have developed a method to control microrobots by placing them in liquid and using magnetic forces. The researchers confined the robots-which are only half a millimeter wide-between two liquids. The application of alternating magnetic fields caused them to assemble into spiky shapes dubbed asters. By applying a second small magnetic field parallel to the surface, the scientists can make the robots pick up, transport and put down nonmagnetic particles. Already, four asters positioned together collected free-floating particles, and another one picked up a glass bead that weighed four times as much as itself.