1) Wash and peel the rhubarb. Cut into 4 cm (1 1/2") lengths.
2) Bring the water and cinnamon bark to a boil. Add the rhubarb and simmer 5 minutes. Remove the cinnamon. The rhubarb falls apart easily if overcooked.
3) Stir with a whisk and remove any rhubarb which sticks to the whisk.
4) Sweeten with sugar. Stir potato starch and water together and stir into the soup. Cook until thickened.
