Physicists often want to smash particles together at high speeds, but sometimes a softer touch is called for. For instance, to study low energy collisions, in which objects don't crack like pool balls but rather stick like Velcro darts, they need sluggish particles. To that end, researchers passed a beam of hydroxyl free radicals (or, water molecules each missing a proton and an electron) through a series of precisely arranged electrodes. Applying a voltage between the electrodes puts the brakes on the hydroxyl radicals, which are then allowed to crash into a second beam of xenon atoms. The resulting ricochets matched up nicely with computer simulations, according to a report in the current issue of Science.