Think of this as a rule of thumb - sooner than go into technicalities like Dative and Accusative etc: "Which one do you want?" Dad asked, holding out three identical model cars, the only difference between them being colour and finish. "What else is there?" my brother Alex answered with another question. "What else do you want?" Dad asked, exasperated. "Which other colour models did the shop have in?" Alex could be trying sometimes. "What colour do you want?" Dad seethed. He'd have to go back to the shop, unless - "Which colours do you like? Come back to the shop with me and see for yourself, then you'll know what you want". Get the picture?