A logical fallacy in which the explanation for a phenomenon doesn't give a reason beyond membership in some category. A valid explanation gives a cause, not just a classification.

Example: "They act that way because they're kids." But why do kids act that way?

To prove the fallacy, accept the explanation and ask why the category exhibits the phenomenon. Lather, rinse, repeat as necessary or until things become circular ("Why do kids do that?" "Because they're kids!" Duh).