Combine fresh grapes, mint leaves, and sugar in a blender. Blend on high speed 10 seconds. Add lime juice and grated ginger; blend again for a few seconds.
Pour the mix through a fine meshed strainer or sieve into a bowl, pressing with a spatula to extract as much liquid as possible. Pour the grape liquid into two large glasses filled with ice(or frozen grapes). Top each glass with half of the seltzer water(or soda water) and stir. Serve immediately. Enjoy!
If you would like to use frozen grapes instead of ice, place the grapes in the freezer several hours before making grape soda.