Bring rhubarb and water to a boil. Simmer, covered, over low heat for about an hour, until rhubarb is good and mushy. Strain mixture through a small sieve, discard mush or reserve for another use. Chill liquid. Fill martini shaker with ice. For each drink, use 2 oz rhubarb liquid and 1 1/2 oz vodka. Shake and strain into glass; garnish with a sprig of fresh mint or drink as is.