How Does a Gravity Bong Work? Physics Explains Negative Pressure

Many a high school smoker has built a gravity bong on the back porch to get stupid high on some mid-grade marijuana, but whatever contraption you rigged up with hot glue and soda bottles probably didn’t hold a candle to Seth Rogen’s sophisticated piece of equipment. Even if you don’t smoke, this video of the comedian using this bizarre device, which climbed near the top of the r/interestingasfuck subreddit on 4/20, is mesmerizing.

But, you’re probably wondering, how does this monster work? While it looks complicated, the answer, as is the case with most ridiculous smoking devices, is physics. And the physics of a gravity bong is actually pretty simple. Rogen’s device, made by the company Stundenglass, uses the negative pressure created as water falls from one chamber into the other to pull air through the burning plant material, replacing the empty space with smoke as the water vacates it. Then, as you turn it back over, the water falling back into the smoke-filled chamber pushes the smoke back out and into your face.

This design is much more slick — and much less of a mess — than the waterfall-style or bucket-style gravity bong you may have built in your garage or dorm room, either of which probably got water all over the place. It also combines the principles of both styles of homemade gravity bongs to pull and push smoke, whereas your trashpipe probably just did one or the other. For anyone who didn’t smoke pot out of discarded food containers as a young person, here’s how gravity bongs work:

One style, which some people refer to as a “waterfall” gravity bong, uses the negative pressure of water falling out of a bottle to pull air through the plant material and fill the chamber with smoke. Much like the Stundenglass pipe, as the water leaves the bottle air must enter to replace it. And the only place for the air to enter is through the lit bowl, which fills the bottle with smoke. The big difference is that instead of falling into another container, the water just exists through a hole in the bottom of the bottle.

It works pretty well, but the problem with this method is that it usually makes a huge mess. First of all, the water has to flow somewhere, and if you’re indoors, you’ll need a sink or bucket to catch the outflow. Second, you need tape or a plug to cover the hole at the bottom of the bottle until you’re ready to rip it. Long story short, it requires a lot of setup and leaves you with a bottle full of smoke that rapidly gets stale as you suck on it and cough your lungs out. The more traditional gravity bong isn’t much better, though.

When you think of a gravity bong, most people will picture the kind that uses a big container of water — as small as a pitcher or as big as a swimming pool — and a bottle with the bottom cut off. This one is a little less likely to spill all over the floor, but if it does, it makes an incredible mess. Here’s how the bucket-style gravity bong looks in action:

This style of gravity bong uses a similar principle as the waterfall, but it accomplishes the end result in a different way. As the smoker lifts the bottle, the falling water level creates negative space in the chamber, which can only be filled with air traveling through the weed. The big difference here is that when you push the smoke-filled bottle down, it once again fills the airspace with water and pushes out the smoke.

So Seth Rogen’s really got the best of both worlds: His Stundenglass gravity bong fills the chamber with smoke like a waterfall bong, but it also ejects the smoke like a traditional bucket gravity bong. As you can tell by the celebrity stoner’s coughs, though, this is not a device for people who place a high priority on comfort.