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Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are some alcoholic drinks shaken while others are stirred, and why is there a difference?
gpt
Hi there, weekday chemist and weekend bartender here!

There are a few "scientific" reasons why some drinks are shaken versus stirred, and one practical reason. First, as others have mentioned, shaking a cocktail with ice vigorously breaks the ice down into smaller pieces which chills the drink much faster/to a lower degree. As you may know, as a general rule clear liquors are best enjoyed ice cold and dark liquors at room temperature (by themselves) - so you *usually* don't shake, say, whiskey cocktails. 

Second, shaking aerates the cocktail. It builds up pressure inside of the shaker tin and forces little bubbles of oxygen into the cooled liquid. This is sometimes required for the desired consistency of a cocktail; shaking makes things frothy and "thicker". Typically cocktails made with juice, eggs, or dairy are shaken.

Drinks that are mostly liquor usually get a nice stir! The larger ice cubes don't melt in the drink as fast and won't water it down. When you're drinking fancy, expensive liquor you want to be able to appreciate the unadulterated color, bouquet, and flavors of the pour. Old fashions and Manhattans come to mind.

Lastly, the huuuuge reason why many bars/bartenders will shake a drink rather than pour - because it waters it down! What else? That means a more sober bar guest and more profit for the restaurant. Don't worry though, most of us would be happy to *correctly* make you a stirred cocktail so long as you're polite!