Our friends, James and Florence, attend choir practice Wednesday evenings, and often head for a restaurant afterwards with their fellow choir members.
Florence soon noticed that every time she had a glass of wine, it was followed by a severe migraine headache. James agreed with her that it might be better if she abstained, and so she did.
On one post choir occasion, however, Florence decided, after some hesitation, to try a different variety of wine. Some time passed with no consequences.
Then she waved happily across the big table where her colleagues all sat and while holding up the empty wine glass announced in a loud voice, "James! I don't have a headache tonight!