Via Crazy Things Parents Say, we have this:

Affirming the Consequent

This is a textbook case of “Affirming the Consequent“. This occurs when an inference is improperly inverted. Saying that A implies B asserts the fact that A is a sufficient condition of B. That is to say when A is true, B must also always be true. It is not, however, a necessary condition. In other words, there are other conditions that might make B true.

More simply put: A implies B doesn’t mean that B implies A.

To give another example of the fallacy to more clearly demonstrate:

If it is grass, then it is green.
I have shaved this horse and painted him green.
Therefore, this horse must be grass.

Moral of the story? Simple inference is a one way street. If you try to go the wrong way down an inference, you’ll end up in a horrible flaming wreck of bad logic.

Also: knowing logic is important, even if it does occasionally damage your sense of humor.