What You Need
4 ounces ruby grapefruit juice
1 1/2 ounces tequila
1/2 ounce Cointreau or triple sec
Moisten rim of a double old-fashioned glass with grapefruit juice. Dip rim of glass in sugar to coat. Fill glass with ice cubes.
Combine grapefruit juice, tequila, and Cointreau in a shaker with 1/2 cup of crushed ice. Shake. Strain grapefruit margarita into prepared glass.