Q. It didn't take much figuring. A single mouthful of grapefruit juice and I knew I had a canker sore in my mouth again. Why do certain foods provoke these painful sores?

A. Though you relate the pain to the juice, the sore was probably there before you took a mouthful of your favorite beverage. Unfortunately, the acid content of grapefruit juice -- as with pineapple, lemon, other citric juices and other spicy foods and beverages -- irritates the ulcerated surface of a canker sore. Many folks discover a canker sore in the mouth when the pain is provoked by the acidic food, though it isn't the food that created the sore.