Grease 2 large baking sheets. Place on wire racks. In heavy 1 1/2 quart saucepan, add first 3 ingredients, stiring constantly. Boil over medium heat. Cook without stirring til mixture is 300 degrees on candy thermometer. Stir in peanut butter, blend well. Remove from heat. Quickly, but gently, stir in soda. Immediately pour onto cookie sheets. Don't spread. Cool for 3 to 5 minutes till cool enough to handle. With hands grasp candy by edges, lifting slightly, pull as thin as possible. Cool. Break into irregular pieces. Store in tightly covered container. Makes 1 1/4 pounds.