A carb is a hole in a hand pipe or bubbler that regulates airflow inside the pipe. Typically, a smoker covers the carb with a finger while lighting the plant matter. Then the carb is uncovered as the smoker inhales to pull fresh air into the pipe and clear it of smoke. Larger bubblers and water pipes don't require traditional carbs. Instead of covering and uncovering a carb, the component bowl in these pipes can be removed after lighting the plant matter in order to introduce fresh air.