"There are three main differences between a martini (or a vodka martini) which has been stirred and one which has been shaken. First, a shaken martini is usually colder than one stirred, since the ice has had a chance to swish around the drink more. Second, shaking a martini dissolves air into the mix; this is the "bruising" of the gin you may have heard seasoned martini drinkers complain about--it makes a martini taste too "sharp." Third, a shaken martini will more completely dissolve the vermouth, giving a less oily mouth feel to the drink.
In a vodka martini, cold is key: a vodka martini that is not ice-cold tastes like lighter fluid. So you shake them. The experience of a traditional martini is more dependent on it being smooth and on not ruining the delicate flavors of the gin. Ergo, one stirs it. Simple enough, no?"
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