Place grapes and 3 tablespoons water in a medium nonreactive saucepan over medium heat. Simmer until grapes begin to fall apart, about 3 to 5 minutes. Strain grapes through a fine mesh strainer into a bowl. Discard solids. Place 1 lemon slice and 1 teaspoon sugar in each of 4 glasses and muddle the lemon. Divide juice from grapes among the glasses and add ice. Fill the glasses with sparkling water and serve.
Variation:
Add a scoop of Concord grape sorbet as a side car for a refreshing treat.