A bong is a sealed chamber half-full with water. A pipe with a bowl on the end goes into the chamber under water level. Another pipe with a mouthpiece on the end enters the chamber and stays above water level. You put tobacco in the bowl, light it and suck on the mouthpiece. This will lower the air pressure in the chamber, causing air to travel from the bowl, through the water, into the chamber and into your lungs. As the smoke enters the chamber through the water the smoke cools down and a large part of carcinogens like tar are filtered by the water. When you remove your thumb from the hole, the smoke shoots straight up into your lungs, so beware!