In a small bowl combine the unpopped popcorn and the water; set bowl aside. Pour the cooking oil into a heavy 3-quart saucepan. Add 2 or 3 kernels of unpopped popcorn; cover and cook over medium-high heat until you hear the kernels pop.
Stir in sugar and, if desired, ground cinnamon. Carefully add the popcorn-water mixture.
Cover and cook over medium-high heat until the popping slows, shaking the saucepan constantly. Quickly remove the saucepan from the heat to prevent the sugar from scorching.
Immediately pour the popcorn into a large bowl. Store tightly covered. Makes about 6 cups.