Water fills the container and stem just up to the carb. The water level should really be at least an inch below the "carb". So that when the bong is tilted the water won't leak. The job of the carb is to regulate where the air will be supplied from. If covered, the air (smoke) will come from the bowl; if uncovered, air will rush through the carb and force out the smoke remaining in the chamber. I won't go through a boring description of every part of a bong--if you don't know, ask someone. The point I want to illustrate is that a bong merely forces the smoke to bubble up through water, thereby filtering it.Most bongs work with this method..however,Gravity bongs work on a different concept.
The anatomy of a conventional bong is *very* simple. It consists of a bowl (and screen), a stem (sometimes optional), a chamber, some liquid, and an opening for the mouth(s)