Mix sugar, vinegar, soy sauce, sherry and ketchup in small bowl, stirring until sugar is dissolved. Dissolve cornstarch in water and add sesame oil. Heat oil in medium-sized skillet over medium heat until hot. Add garlic and stir for one minute. Slowly add sugar mixture, stirring until the liquid comes to a boil. Turn heat to low, add cornstarch mixture, stirring, until sauce begins to thicken. Slowly add remaining water while stirring. When mixture is smooth, remove from heat. Makes 3/4 cup sauce. Can be refrigerated 4-5 days or frozen for up to two months.