From gluten-free beers to fruit-infused brews, so many trends come and go in the brewing world that it’s hard to keep track of—let alone try—them all. Right now, one of the hottest crazes is nitrogenated beer, a.k.a. “nitro beer,” inspired by some classic stouts, such as Guinness, which has used the brewing method for decades. Wondering how nitro beer is different from regular carbonated beer, but too embarrassed to ask your bartender? The answer lies in the gas used in the carbonation process. 

Regular beers are brewed and pressured with carbon dioxide. Nitro beers, however, contain a mixture of about 70 percent nitrogen and 30 percent carbon dioxide.

Nitrogen is largely insoluble in liquid, and it forms tiny bubbles, which are smaller and smoother than CO2 gas bubbles. These bubbles give the beer a thicker texture and a larger, creamier head.

In contrast, regular carbonated beer may taste hoppier and more aromatic, and it won’t feel as smooth in your mouth. The head will also be less dense, and its pour will be thinner.

One final, key difference between the two types of brew is their bubbles. Pour two side by side, and look at the glasses. A non-nitro beer’s bubbles will rise up the sides of the glass, whereas a nitro beer’s bubbles will be more concentrated in its center.

There’s a lot to learn—and love—about beer. For more answers to questions about your favorite brews, including Guinness, check out mental_floss’s Big Questions channel—preferably with a Guinness in hand. (But please remember to drink responsibly.)