Can a splash of spirit transform an entire cocktail? It depends on how it’s poured.

Take the Penicillin, a mix of blended scotch, honey-ginger syrup and lemon juice that’s shaken, strained, the peaty scotch floats in a single layer on top—the nip of liquor suffuses the entire drink, ensuring that each sip starts out with Islay’s smoky, funky flavors and finishes with the sweet-and-sour base.

The Penicillin is one of the best-known cocktails that uses a float, but the technique can be broadly applied, allowing a small amount of high-cost or intensely flavored booze to have an outsized impact on a drink.

At Drink cocktail bar in Boston, general manager Ezra Star uses floats to put “the sharp sting of spirit first.” The cocktail below cools the sting.

She might float Martinique rum on a mojito to add complexity to its clean, minty flavor. Or she’ll modify a Death in the Afternoon, a cloudy Champagne cocktail, by floating the absinthe on the gold bubbles to create a bright green layer that mixes with the sparkling wine as one drinks.

Floats work because of differences in density. Denser liquids sink; less dense liquids tend to float on top of them. To predict an ingredient’s density, consult a chart of specific gravities (the density of a liquid compared to water), like the one on cocktailhunter.com.

As a general rule, viscous, sugary items, like grenadine or syrupy liqueurs, sink. High-proof spirits with a lighter body (and no added sugars) float more readily. In our New York sour, we upped the density of a whiskey sour base by using rich simple syrup (2:1 sugar to water), making its red wine float more foolproof.

Floating can still be tricky. Star recommends holding an overturned spoon as close to the liquid as possible, bracing the tip against the edge to steady it. Using a measuring cup with a spout (we like OXO’s stainless-­steel angled jiggers), slowly pour the liquid onto the drink below.