At my first bartending job, at a pub in the English countryside, it took me a while to master the two-step pour of a pint of Guinness, from angling the glass to topping it off so that the foamy head was the ideal thickness. As the pint sat on the bar, waiting to be imbibed, mocha-colored bubbles raced down the sides of the glass as the stout settled. I developed deep respect for the process, as well as the cool creaminess of the first sip.

So the first time a customer asked me to swirl a shamrock onto the top of that foam, I gaped back at him for a hot moment. I thought, why on earth . . . ? The same request might have also earned the customer a disdainful look in, for instance, Galway. And rightly so, because what’s happening inside the glass is much more interesting than any kind of added flourish on top.

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What makes Guinness so unique, besides its velvety scrumptiousness, is that every exported keg calls for nitrogen-heavy carbonation from a coupler that discharges a tiny bit of the gas into each pint, which helps create that fine, creamy head. (In cans of Guinness, this same function falls to a round nitrogen widget.) After a pint of Guinness is poured, a wellspring of bubbles constantly rises through the middle of the liquid to the head, while a concurrent downstream occurs along the sides of the glass. This is the result of a complicated imbalance between gravity and glass shape (smaller at the bottom, wider at the top) and the minuscule size of the nitro bubbles.

So on St. Patrick’s Day, after you’ve downed a pint or two of Guinness and begun to think it’s quite a magical drink, you’re mostly right. This beer hits all of the finer points: flavor, texture, and movement — and all without a bartender spending a few extra seconds on a decorative shamrock. Slainte!