Put about 3 shots of vodka into a cocktail shaker full of ice.

Next, squeeze all the juice from a large grapefruit (Texas Ruby are best) into the shaker (it's ok if some of the pulp goes into the shaker too). Fresh squeezed is a must - this is a good way to use up those old grapefruits you haven't eaten.

Shake vigorously and serve!

Makes about 2 martini glasses full.