Mix the corflour with the orange juice until smooth. Put all the other ingredients except the prawns into a saucepan, add the orange juice and cornflour and bring to the boil, stirring regularly until thick and smooth. If you want a reddish-coloured sauce add the tomato puree and, if you like the flavour, add the crushed pineapple, or both. Put in the prawns and heat through for just two minutes, Don't cook them too long or they'll turn rubbery. Serve with plenty of rice and stir-fried beansprouts.