When I say "citric acid," what comes to mind? The weak organic acid? The compound involved in the Krebs Cycle, powering most metabolic reactions? [Five extra points if you paid attention to this part of biochemistry class]. The powdery tart stuff on Sour Patch candies, the ingredient in meat tenderizer, and in canning recipes?
Citric acid is all of these things, and is used for more widely varied purposes than I can list here. Sometimes also called "sour salt," Citric Acid is a natural acidic ingredient found in all citrus fruits [lemons, limes, oranges, even some berries]. Citric Acid resembles salt, fine and a little powdery, and some people on low-sodium diets use it as a salt substitute.
Citric Acid has many uses in the kitchen. Often cited as a canning aid, Citric Acid helps prevent fruits from discoloring. Citric Acid works as an emulsifying agent in homemade ice cream batter to keep fats from separating, and in caramels to prevent the sugar from crystallizing.
Use citric acid in place of liquid lemon juice or vinegar where sourness is needed, but the flavor or liquid is unwelcome in the recipe. Citric Acid can assist in home wine-making as a boost for fruits that have weak acidity of their own, or in home beer-brewing to modify brewing water.
Citric Acid also can be used to tenderize meats [it breaks down meat proteins - part of that Krebs Cycle thing], and in ripening homemade mozzarella cheese.
If you have some leftover Citric Acid, after making all these delicious homemade concoctions, you can also use the stuff to make fizzy bath crystals, and to synthesize the explosive HMTD. What wacky stuff!