The instructions on a can of Campbell's Tomato Soup tell you to use one can of water to one can of soup. Doing so will yield a reddish fluid very similar to watered-down ketchup (or catsup, I guess). If this is your thing, fine. If not, what you really want to do is use a can of milk instead. This yields a rich, creamy soup that is absolutely delicious.
Things you can put in tomato soup that taste good:
Nothing makes a rainy day at home more enjoyable than a big jigsaw puzzle, some tomato soup, and a grilled cheese sandwich to dip in it. Mmmm.