Physics is a topic best not contemplated under the influence of alcohol. If the room looks like it’s spinning or a single image starts to look double, you can be sure that what you’re seeing is not the way the world really is. And yet even before your very first sip, there’s one screwy bit of physics that’s hard to deny: the bubbles in your glass of stout appear to be sinking.

Bubbles, of course, are supposed to move up, not down, for a very basic reason: the gas that fills them is lighter than the surrounding liquid. Just like a balloon filled with lightweight helium must rise in heavier oxygen-nitrogen air, so too must the swirl of bubbles in your beer move toward the top of the glass. Yet judging by what we see, a lot of them don’t. A new study in the American Journal of Physics, authored by William Lee, professor of industrial mathematics at the University of Huddersfield in West Yorkshire, England, explains why.

The falling-bubbles phenomenon is observed most conspicuously in stouts—strong, dark beers made with roasted barley or hops—and it’s the nature of their dissolved gasses that makes the difference. In lighter beers, the gas that creates the fizz and foam is entirely carbon dioxide. In stouts, it’s a mix of carbon dioxide and nitrogen.

Nitrogen dissolves in liquid less easily than CO2 does, which means that while the gas content of stouts is lower, the pressure is higher. Most important, the bubbles in a stout are smaller than those in a lighter beer—an average of a tenth of a millimeter across compared to a millimeter or more. Smaller bubbles are less buoyant due to the lower amount of gas they contain. But they still ought to move in only one direction, and that’s up.

Lee explains that in the early years of sinking-bubble research (yes, the investigations have been going on for a long time), there was some question as to whether the phenomenon was occurring at all, or, as he wrote in his paper, if it is merely “an optical (or alcohol induced) illusion.” In 2004, a Stanford University study, straightforwardly titled, “Do Bubbles in Guinness Go Down?” relied on videotape analyses to determine that the bubbles under study were indeed moving south. Other research used computer models to confirm that finding, but they offered little to explain why it was happening.

“The ability to reproduce a phenomenon in a simulation,” Lee wrote, “does not always lead to a better understanding of that phenomenon, any more than observing it in the real world does.”

In his new paper, Lee went old-school, eschewing computer models and relying on the modern-day equivalent of pencil-and-paper ciphering to develop a clear mathematical model for what is happening in the glass. His equations include multiple variables, including the volume of the bubbles relative to the volume of the liquid, the velocity of the bubbles, the velocity of the beer (which is not stationary) and the overall pressure in the entire bubble-beer-glass system.

After all of that and more was factored together, Lee’s most important finding is that — never mind the evidence of the videotape, the computer models and your own eyes — the sinking bubbles actually are just an optical illusion. Sort of. Beer bubbles will indeed always rise relative to the liquid in which they form, but some of that liquid is circulating downwards relative to the glass. Draw an imaginary line in the liquid and the bubbles rise past it; draw it on the glass and they sink below it. The phenomenon is especially pronounced if the beer is poured into the so-called tulip glasses in which stouts are typically served—glasses with a narrower base and a wider mouth.

Once a beer is poured, Lee explains, bubbles ascend in a more or less straight line, even as the tulip glass steadily widens. That leaves a region of beer close to the walls of the glass with a lower concentration of bubbles. Beer with less gas is denser than beer with more, and that region of beer thus sinks, carrying the nearby bubbles with it, relative to the glass. As the beer settles in the narrower bottom, the bubbles, which are now more concentrated, rise up en masse, and the cycle repeats itself.

“If you were to pour [stout] into a glass with walls that sloped inwards,” Lee wrote in an email to TIME, “then things would happen the other way round. As bubbles rise they would crowd together at the wall creating a lighter, bubble-rich fluid which would rise upwards.”

The bubble-beer-glass system, like most dynamic systems, ultimately exhausts itself. Left long enough, the foam would vanish, the gas would dissipate and the stout would go still and flat. Of course, most stouts—or at least most good, cold, dark, rich, delicious stouts—don’t hang around nearly that long. The human drinker intervenes—and then the human bartender begins the process anew.

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Write to Jeffrey Kluger at jeffrey.kluger@time.com.