It’s the moment you’ve long dreaded: You’ve invited your mustachioed, speakeasy-going friend over for dinner and he’s asked for a cocktail. He watches incredulously as you pour gin, vermouth, and Campari into a cocktail shaker, his voice slowly trailing off from his story as the ice slams back and forth in your shaker. As you strain the slushy mixture into a glass, you’re forced to listen to a tirade about when to shake and when to stir, and you can’t help but think to yourself: Does it really matter?

Well…yeah. He’s not wrong. 

Every cocktail is comprised of a few key ingredients. You need booze, or it isn’t a cocktail. You need modifiers (like citrus and syrups), or it’s just a shot or neat pour of spirit. You also need water, which almost always comes in the form of ice. Water does many things for alcohol, but one of the best things it does is to lower the relative proof from something mouth-numbingly strong to a more enjoyable level. Think of Picasso’s Guernica: At nearly 12 by 27 feet long it’s enormous, but if it were on something the size of an iPhone you would never be able to see any of the detail. That’s what water does: It adds more canvas. If you consider the fact that most cocktails are about 3 to 3 1/2 ounces of ingredients, but can be reliably called upon to fill 4 1/2- to 6-ounce glasses, you come to realize that somewhere between a quarter and a third of your finished cocktail is water. So the kind of water you’re adding -- and the way you add it -- will end up changing the character quite a bit.

A great rule of thumb that will rarely ever steer you wrong is this: Stir spirits. That is, if your cocktail is all liquor, stir it. This includes the martini, Manhattan, old fashioned, negroni, and all of their variations. The technical reason for this is simple: They all contain alcohol and have relatively similar densities. They can be incorporated very efficiently by just stirring them together, and if you mix Campari, vermouth, and gin in a glass and let it sit for a week, every part of that mixture will be an undiluted negroni.  

When you stir, the ice should be a combination of very dense, large pieces and smaller chips. The best way to achieve this is to put your cocktail in the mixing glass, add large pieces of ice until full, then crack smaller pieces of ice into the glass with your bar spoon until it has filled in the gaps left by the large ice. Stir vigorously but smoothly until your cocktail is properly diluted, which depending on environmental factors (was the mixing glass chilled? Did the ice just come out of the freezer? Are you in Death Valley?) can take anywhere from 5 seconds to a minute. If you’re using good ice cubes from right out of the freezer, shoot for 25 seconds. (Alternatively, just stir whole cubes, without cracking them first, for slightly longer.) Despite popular belief, shaking doesn’t get cocktails colder -- as long as you’re taking the time to stir correctly. Stirring will yield an icy, dense, and silky cocktail, while shaking will yield a frothy, light, and crisp cocktail.

The major difference between shaking and stirring is texture, and the reason the textures differ so much is because of the ingredient that you add while shaking that you don’t add while stirring: air. Shaking, if you’re doing it right, is a violent and dynamic process. The ice cubes are shattering into miniscule shards and adding tiny bubbles to your drink while slamming back and forth in the shaker. This is extremely important when you’re using citrus as an ingredient in your cocktail, because without the lightness that shaking imparts to the drink, the acidity of the citrus can be unpleasant and intense instead of light and refreshing. And citrus juice and alcohol are very different densities, which means they don’t mix easily. (If you mixed gin, lemon juice, sugar syrup, and club soda together and left it for a week, you wouldn’t walk back to find a Tom Collins -- you’d find a boozy mixture sitting on top of suspended pulp.) 

The ice in the shake is just as important as it is in the stir: Use large cubes or pieces of very cold, dense ice. (If you’re using standard cubes, a good rule of thumb is 5 cubes.) Shake hard and fast. In my experience it is a fact that you can’t shake a cocktail too hard, merely too long. Again, it’s going to take a little practice, but a good rule of thumb is to shake as hard as you can for 8 to 10 seconds. The larger the chunk of ice in the shaker, the longer you have to shake. And keep in mind that egg white drinks require additional shaking, because you’re adding yet another level of density to the mix. 

So: Shake for citrus, stir for spirits. Those of you who have never had a properly stirred martini, give it a try -- you might surprise yourself with the results.