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Soda and citrus flavors were combined in 1938 to create a grapefruit-lemon soft drink that would later inspire Coke to make Fresca. Fresca was popular when it was introduced in the 60s since it was artificially sweetened and contained no calories. That was back when diet drinks were just catching on. Nowadays just about every soda comes in a diet version, and Fresca sales have slipped, despite a tweaking of the formula in the early 90s.
Squirt continues to hold on to a loyal cult following, with many who claim the soda is the only true cure for a hangover. To clone it, just add real bottled white grapefruit juice, along with a little Kool-aid mix for a lemony zing, to the simple syrup recipe. Chill the syrup and soda water until cold and get ready to make a dozen cups worth of citrus soda at home.
Source: Top Secret Recipes: Sodas, Smoothies, Spirits & Shakes by Todd Wilbur.