4) You stir when you should shake, and vice versa
All drinks are not created equal — some are neat, some shaken, and some are stirred, though preferably not with a finger. Vigorous shaking provides aeration and also dissolves more water, which can be helpful with higher-proof alcohols. Stirring prevents over-dilution for more delicate drinks, which is why a proper dry martini is usually recommended stirred (though how many times depends on the size of your ice). Either way, a bar spoon, a mixing glass, and a shaker set are all worthwhile investments.