Food preferences can vary among individuals, most likely from early experiences that are often subconscious. Some don't like 'smushy' foods, some don't eat greens, and in this case, maybe she simply doesn't want to eat lettuce. If someone consistently refuses solid food, but can swallow liquids, there may be an underlying problems, such as esophagitis, that can lead to this food preference (i.e., everytime they eat a solid food, maybe it gets stuck in the throat or chest, so they learn to avoid these foods). But for an single food item, most likely this is simply a food preference. Sometimes repetitive introduction in a non-threatening environment, with others eating same foods, can lead to eventual acceptance of the food.