A beekeeping smoker, or just called a smoker, is a tool to produce smoke in the attempt to calm down bees. The basic principle is smouldering some fuel, which is the flameless type of combustion of the fuel.
Calming bees using smoke has been practiced since long ago in ancient times. Although it has been practiced that long, scientific studies could not satisfactorily explain why and how the bees can be calmed down by smoke.
A clear explanation is that smoke can disguise alarm pheromones. These pheromones are released by the guard bees signaling the presence of an intruder. Injured bees during an inspection of a beekeeper could release these alarm pheromones as well.