Why are "fish" paraphyletic?

Q: Can you please explain question 3 on midterm 2? It asks why fish are a paraphyletic group.
The answer is because they don't include tetrapods, but I don't understand why tetrapods have anything to do with this.

A: To answer this lets look at why fish are not a monophyletic group. Such a group contains an ancestor and ALL of it's decendents. A monophyletic group of "fish" has to include all creatures that descended from fish (and therefore from the common ancestor of all fish). Tetrapods emerged from one type of fish. So if we don't include them in the clade of "fish", then fish are defined as a paraphyletic group (includes an ancestor but not all of its decendents).