The marine bacterium Vibrio harveyi glows when near its kind. This luminescence, which results from biochemical reactions, is part of the chemical communication used by the organisms to assess their own population size and distinguish themselves from other types of bacteria. But V. harveyi only light up when part of a large group. This communication, called quorum sensing, speaks for itself here on a lab dish, where more densely packed areas of the bacteria show up blue. Other types of bacteria use quorum sensing to release toxins, trigger disease, and evade the immune system.