a, Many synthetic reactions are performed using polymeric beads. Each bead is typically 75–150 micrometres in diameter, so many hundreds of beads are used in each reaction. (Beads are not shown to scale). b, When added to a solution of gadolinium chloride (GdCl3) in a magnetic field, such beads levitate. The height at which they levitate depends on their density, which varies during the course of a reaction. c, Mirica et al.4 use this effect to follow the progress of solid-supported reactions. In their method, beads are removed at intervals from a reaction, and changes in the height at which they levitate are observed. When no further change in levitation height is observed for successive beads, the reaction is complete.