3 1/2 oz of grapefruit juice
1 oz of Vodka
1 oz of creme de cassis (or blackcurrant-flavored liqueur)
1 oz of Midori melon liqueur (or watermelon liqueur)
Pour the vodka, creme de cassis and vodka into a cocktail shaker half-filled with ice cubes. Add grapefruit juice, and shake well. Strain into a highball glass, and serve.