CUT UP THE WATERMELON FLESH AND PURÉE IN A BLENDER, PREFERABLY AFTER REMOVING AS MANY SEEDS AS POSSIBLE FIRST. Strain the watermelon pulp.

COMBINE THE WATERMELON JUICE WITH THE MINT AND SUGAR IN A BLENDER, THEN PURÉE. Transfer the mixture to a plastic airtight container and place it in the freezer for about 4 hours, until the mixture is completely frozen.

TAKE THE CONTAINER OUT OF THE FREEZER, BREAK THE FROZEN MIXTURE INTO PIECES AND RETURN THEM TO THE BLENDER TO TURN THEM INTO SLUSH (THE CONSISTENCY OF MELTED SNOW). Add the sparkling water and serve with straws in tall glasses.