Here, tangy raspberries and tarragon transform into a shrub--a sweet vinegar-based syrup that's been around since Colonial times. Mix into sparkling water for a refreshing sip. You'll make enough shrub to mix up 20 drinks.
Combine raspberries, sugar and tarragon in a large nonreactive pot, such as stainless steel. Let stand at room temperature, stirring occasionally, for 1 hour.
Cook the raspberry mixture over medium heat until starting to simmer, about 5 minutes. Add vinegar and continue cooking, stirring occasionally, until it comes to a bare simmer (do not let it boil), 3 to 5 minutes. Remove from heat. Cover with cheesecloth or a clean kitchen towel and let stand at room temperature for 24 hours.
Remove and discard the tarragon. Strain the mixture into an airtight container. (If desired, save the pickled berries for another use; refrigerate airtight for up to 1 week to use in salads.) Refrigerate the shrub until cold, at least 1 hour and up to 1 month.
To make each drink: Fill glass with ice. Add 1 1/2 tablespoons shrub and top with sparkling water. Stir once and garnish with fresh raspberries and tarragon, if desired.
To make ahead: Refrigerate shrub (Steps 1-3) for up to 1 month.