While there are lots of fun styles and colors available these days, most gravity bongs use the same simple principles. Many glass gravity bong use two pieces, an outer "container" that holds the water and an inner chamber that you pull up during your hit to create the pressure. The water acts as a barrier so air can only pass through your bowl when you are lifting up the inner chamber leading to thick and powerful smoke or vapor.
Start by filling up the outer chamber with water (we like to use as much as possible in our gravity bong glass) and then insert the other glass piece. Make sure to not put your bowl on until after both pieces are in the water or the air pressure will blow out your flower! Then just light your bowl as gently pulling up the inner chamber and watch it fill with smoke. Once you've picked up as much as possible (make sure not to pick it up out of the water or you'll lose smoke out of the bottom), remove the bowl, place your mouth over the opening and slowly (or fast) push the chamber back into the water forcing the smoke out into your mouth. Repeat as many times as you'd like!