Carbonation is that special “ingredient” in beer that makes it so darn delicious. It enhances the beer’s aroma, creates an enjoyable texture in the mouth, dries out your palate so you yearn for another sip, and helps balance a beer’s flavor. But if you’ve ever been served a pint at the bar and seen any of those little wondrous carbonation bubbles anchoring themselves to the sides of your glass, send the pint back immediately, because your glass is filthy.

That’s right, in addition to being one of beer’s most incredible ingredients, it’s also a great detector of dirty glassware. This is because carbonation bubbles colonize around forbidden funk on the inside of a glass, whether those foreign materials are oils, dish soap, or food residue. And this residue can not only ruin the taste of your beer, it’s also not something you want to ingest, especially if that residue is lingering detergent.

The inside surface of a beer glass should be nice and smooth, allowing the carbonation bubbles to rise to the top in a fluid motion. If some of them instead decide to hang out on the inside of the glass’s surface, ask for a new pour. Or, if it’s your own dishwasher that’s the culprit here, consider rinsing and wiping your glass with a microfiber cloth before pouring.