My roommates often organize events we call “presentation nights” in our dorm, where a few people give presentations on topics they’re passionate about — topics such as the best parking lots, favorite TV shows, random quizzes, musicals, unique high school experiences and whatever else we find interesting. Everyone listens attentively and asks questions at the end, just like in a formal presentation — except these are often hilarious. There are no grades or pressure, just a shared enthusiasm for oddly specific topics.
During one such presentation night, two of my roommates debated an unexpected yet strangely compelling question: Is peanut butter a soup? The discussion was intense, with arguments thrown back and forth, but in the end, we didn’t reach a definitive answer. I knew I couldn’t just leave it unresolved — I had to do some research.
The first thing that came to mind was an experience I had while traveling. I once tried to carry a jar of peanut butter onto a plane, only to have it confiscated. According to the Transportation Security Administration, carry-on liquids must be under 3.4 ounces. Peanut butter is considered liquid, and since it exceeded that limit, it was seized.
To understand why peanut butter is considered a liquid, we first need to define what makes something a liquid. The key is understanding fluids. A fluid is any substance that flows continuously when a shearing force is applied. There are two main types: Newtonian and non-Newtonian fluids.
Newtonian fluids, like air and water, behave predictably. Their viscosity, or resistance to flow, remains constant at a given temperature. If you pour a glass of water, it moves smoothly whether you tip the glass a little or a lot.
Non-Newtonian fluids, on the other hand, don’t behave as consistently. Peanut butter is one of them. It doesn’t pour like water, but it will move if enough force is applied. Think about stirring a jar of peanut butter: if you stir gently, it remains thick and resistant, but if you stir harder, it becomes looser and easier to spread. The same thing happens when you apply heat — peanut butter on warm toast spreads more easily, sometimes even dripping.
Fluids are further divided into two main categories: liquids and gases. Peanut butter falls into the liquid category because it takes the shape of its container but doesn’t compress. If you squeeze a peanut butter sandwich, the peanut butter doesn’t shrink, rather it just oozes out.
The verdict I’ve reached is that while peanut butter isn’t technically a soup, it is a liquid. And interestingly enough, it shares a key characteristic with soup: both are non-Newtonian fluids. In a way, that makes them part of the same family; an odd one, but a family nonetheless.