Mix the sugar, vinegar, maraschino cherry juice, and pineapple juice in a saucepan over medium-low heat, stirring to dissolve the sugar.
Place the cornstarch in a small bowl, and stir the water into it to make a smooth paste. Stir the cornstarch paste into the juice mixture, and bring to a simmer. Stir constantly until the mixture thickens, about 1 minute.
