A bowl is the structure that holds plant matter. Smoke is produced by lighting a bowl's contents with a flame. Some bowls are built into a pipe, like the push bowl on a GRAV® Steamroller. Push bowls are indentations in a pipe formed by pushing a tool into the glass while it's soft. Other bowls are separate components with standardized joint sizes intended to fit a variety of water pipes. Component bowls can be removed while smoking in order to clear the water pipe with fresh air. They can also be more easily cleaned and replaced when broken. Component bowls are known elsewhere as "slides", due to the fact that they're slid in and out of a water pipe during use. A bowl is also a unit of measurement, referring to one bowl's capacity for plant matter (as in "hand pipes should be cleaned after every tenth bowl smoked").