Fill a shaker with ice cubes. Add lemon juice, gin and simple syrup. Shake and strain into a chilled highball glass. Top up with soda water. Garnish with lemon.
The first printed reference to a fizz (spelled "fiz") is in the 1887 edition of Jerry Thomas' Bartender's Guide, which contains six fizz recipes. The Fizz became widely popular in America between 1900 and the 1940s. Known as a hometown specialty of New Orleans, the Gin Fizz was so popular that bars would employ scrums of bartenders working in teams that would take turns shaking the fizzes. Demand for fizzes went international as evidenced by the inclusion of the cocktail in the French cookbook L'Art Culinaire Francais published in 1950.