After one has smoked a pipe, relishing the sweet haze of tobacco smoke, and imagining oneself as Sherlock Holmes or perhaps J. R. "Bob" Dobbs, there will be something left in the bowl. This substance, made up of ash, carbon and unburnt tobacco, is known as dottle.
Dottle can be removed by gently shaking one's pipe, or if stubborn, holding the bowl in one hand while gently tapping the bottom with the other. Do not hold a pipe by the stem and bang it on a hard surface to remove the dottle. This is a very bad thing.