Soak the tapioca in the water. Add to it the butter, melted, a little salt, half the sugar and the milk. Peel and core six sour apples, filling them with the rest of the sugar with which the juice of half a lemon and the grated peel of a whole lemon have been mixed. Put the apples into a deep pudding-dish. Pour the tapioca and milk round them. Bake an hour.