It’s rather easy to turn a fruit juice into an alcoholic beverage. All you need to do is add one of the following: vodka, gin, rum, tequila, or whiskey. In addition, you can add a liqueur to sweeten the drink. Bartenders often refer to sweet, fruity, delicious drinks as “kiddie drinks,” meaning that kids (as in college kids of legal drinking age) might prefer them to the more adult-like drinks; scotch on the rocks, whiskey straight or neat, or a traditional martini.
These “kiddie drinks”, however, are often drank by adults and can be considered to be somewhat healthy due to the fruit juices in them. The most popular fruits used are orange, grapefruit, cranberry, tomato (technically a fruit), and pineapple. Pineapple is generally the thickest of all fruit juices, and the best option for masking the taste of alcohol mixed with it (generally vodka). Adding vodka to any one of these fruits will be the easiest way to make your own alcoholic fruit juice. (Of course, once you add alcohol, it is no longer considered a pure fruit juice).
There are many levels to creating an alcoholic fruit drink, namely the total number of mixers you include. For example, vodka and orange juice is a Screwdriver, and if you add cranberry juice, it becomes a Madras. And if you add some peach schnapps, it becomes a Sex On The Beach. Just vodka and cranberry would make a Cape Codder, and if you add grapefruit to that, you get a Sea Breeze. And if you want to impress your guests, feel free to add an actual slice of grapefruit to the cup, as a garnish of sorts.
It should be stated that fruit juice wasn’t always used for mixing. Just a few decades ago, they did not have apple martinis and mango martinis. A martini was originally made using just gin that was stirred with ice, then poured into a chilled martini glass which contained just a drop of dry vermouth, and garnished with one or two olives, for just a hint of added flavor. Now, we use real fruit juices, adding much more flavor and thickness to mixed drinks.
When you put booze in a hip flask, you should not put fruit juice or any mixers in there. Rather you would normally pour from the flask into a glass of whatever mixer your prefer, if you are not drinking the spirit “straight” and straight from the flask (pun intended). Although technically a stainless steel flask can handle fruit juices, it is very hard to clean, and therefore advised to keep one’s fruit juice mixer in a separate cup or bottle, and leave the hard stuff for the flask.