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. If particles are lost, the asters can reshuffle to self repair. These microrobots could enable fabrication of precisely designed functional materials in new ways; they can handle items more delicately than mechanical micromanipulators.