We’re eating dinner – main course mussels, which Teresa cooked because she thought Lillian like them. Lillian isn’t eating much. She says she has a bit of a headache.
Teresa: “Then you should eat up your mussels. They are good for headaches.”
Lillian, plainly skeptical: “How do you know?”
[Aside: to me this seems another milestone – a seemingly clear cut case of challenging the speaker to provide rational justification.]