Plant scientists say tomatoes are fruits because they contain seeds. However, most nutritionists think of tomatoes as vegetables because they contain so few calories compared to other fruits. For example, a medium apple contains about 80 calories while a medium tomato contains only 20 calories.
Tomatoes aren’t the only fruit most people think of as vegetables. Cucumbers, peppers, eggplant and squash are also considered fruit because they contain seeds. While this is a very simple explanation of the differences between fruits and vegetables, in most cases it works. Think about foods that contain seeds; most are considered fruits by plant scientists. Defining vegetables so simply is not that easy. Vegetables can be edible roots (e.g. carrots), tubers (e.g. potatoes), and stems (e.g. celery), flowers (e.g. broccoli) or leaves (e.g. lettuce).