1. Place rhubarb, sugar, and 1 1/2 cups of water in a saucepan. Bring to a boil, then turn down to a simmer. Cook for about 15 minutes, until the syrup is bright pink.
2. Turn off the heat and allow to cool. Strain syrup into a large jar.
3. To make each soda, measure 1/4 cup rhubarb syrup into a glass. Add enough sparkling water to fill the glass half full. Stir to mix, then add ice.