Make Lime Syrup:
Scrub 4 limes under running water and pat dry. Using a zester or sharp vegetable peeler, remove the zest from the limes. Squeeze juice from limes and set aside to make the Fizzes.
In a saucepan, combine 2 cups sugar and 2 cups water. Over medium heat, bring to a boil, stirring until sugar dissolves. Add the lime zest and simmer, not stirring, for 10 minutes. Remove from heat and let cool. Syrup can be made ahead to this point and refrigerated.
To make 1 Fizz:
In a tall glass, combine 2 tablespoons lime juice and 2 tablespoons Lime Syrup. Stir. Add ice cubes, then top with sparkling water to fill.
2 cups sugar