Combine 1 ½ ounces Patrón Reposado with 3 ounces fresh grapefruit juice, ½ ounce fresh lime juice, and ½ ounce agave syrup in an ice-filled cocktail shaker.
Give it a good jiggle until the shaker is frosty.
Pour the paloma base into a tall glass filled with fresh ice.
Top the drink with sparkling water.
