Combine cola soft drink, vinegar, and sorghum in a 4-qt. saucepan, and bring to a light boil over medium heat. Cook, stirring occasionally, 1 hour to 1 hour and 10 minutes or until mixture coats a metal spoon and runs off in a fluid stream. (Mixture should be reduced by two-thirds.) Remove from heat, and cool completely (about 1 hour). Store in an airtight container in refrigerator up to 2 weeks.