"I Already Know That!"

Collin's new phrase the last month has been "I already know that" when you tell him something or answer a question he asks.......even though he does NOT know the answer.  Why would he have asked the question?  For instance, "Mommy, why do I need to brush my teeth".  I tell him "So you don't have stinky breath (he does not like stinky breath so this works every time) and that his doctor said he needs to".  Before I finish, he is responding "I already know that".  Well, then why did you ask the question?!?!?!