To smoke a bong, the smoker must inhale in the bong so bubbles containing smoke begin to come from the stem. Once the bong has a fair amount of smoke built up, the stem is separated from the bong and the remaining smoke will go into the smoker’s lungs.
However, a 2000 NORML–MAPS cannabis study found that “water pipes filter out more psychoactive THC than they do other tars, thereby requiring users to smoke more to reach their desired effect”.[src]
The Grasshopper 