Who vs. Whom

The other day I saw a bumper sticker, in the shape of the paw, that said "Who rescued who?" Can you figure out why this is wrong?

People often get confused when deciding between who and whom. This is how I remember it: If you can replace the word with "him," it should be whom. If not, it should be who. Using the bumper sticker as an example, the implication is that the owner rescued the dog, but the dog also rescued the owner. So we would say, "he saved him." Since the first one is "he," that is correct to be who. But the second is "him," so that should be whom. In other words, "Who saved whom?" Hope this makes sense. If not, leave a comment on the blog and we'll discuss it more.