Make a rich chicken stock with leftover chicken bones or giblets. If it is not strong enough, add a stock cube. Season with salt and pepper.
Add the rice, pastina, or tapioca to the boiling stock and simmer until tender. Beat the egg yolks or whole eggs and add the lemon juice, beating constantly. Add a ladleful of the soup to the egg mixture and beat well. Pour this back into the pan slowly, still beating constantly. Keep the heat under the pan very low, and cook the soup gently, stirring all the time, until it thickens. On no account allow it to boil, or the eggs will curdle. Taste and adjust seasoning, adding more lemon juice if necessary.
Garnish, if you like, with chopped parsley or chives, and serve immediately.