A bowl holds plant matter that you smoke or vaporize. Smoke is made by igniting a bowl's contents with a flame. Some bowls are built into a pipe, like the push bowl on a spoon. 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 varying joint sizes intended to fit a variety of water pipes. These bowls can be removed while smoking to clear the water pipe with fresh air. They can be cleaned easily and replaced when broken. Component bowls are known as "slides," because they're slid in and out of a water pipe during use. A slide bowel typically does not fit into a sized downstem, and instead, has a grommeted area near the bast of the bowl. A bowl is also a unit of measurement, referring to one bowl's capacity, as in, "Damn, I can't believe we smoked ten bowls."