/Qual e`/ is used when you are asking within a set of things that is clear from the context, and that share something in common. / Cosa e`/ is when the set is more or less completely undefined. -- qual e` la stella piu` luminosa ? (set of stars) -- cosa ti piace di piu` da fare nel tempo libero ? (the set of possible activities is not defined) -- cosa cucini oggi ? (a possible very vast set to choose from) -- quale cibo cucini oggi ? (set of /food/)