Add grapefruit juice, lime juice, vodka, triple sec, and ice cubes to a cocktail shaker and shake vigorously to combine.
Add ice to serving glasses and then add a Tablespoon or two of tonic water, so the alcohol doesn't settle at the bottom.
Next, pour the cocktail mixture evenly into the glasses, leaving a little room on top for more tonic water. Top off with tonic and garnish with fresh slices of grapefruit and lime. Serve immediately. Yum.