I picked up this quip from an episode of CSI. I can't remember it word for word but this is how I would have used it;

"To be retarded is defined as being held back, or unable to move forward. It seems to me that you are the one who is retarded."

1982 Pontiac Trans Am...K.I.T.T. in progress. "It looks like Darth Vaders bathroom in here..."