Possibly the greatest comeback ever devised. Although it seems childish it was originally used to almost parody the usual immature responses in mature arguements with immature people.
It was brought to popular use from the popular Machinima, Masterchief Sucks At Halo 3/Arby and the Chief

A retort to "you are", the the mid-level retaliatory responce.

No you, Also known as No U.

Dumbest comeback known to man, YET most effective.

It reflects Teh PWNAGE (Look that up) back to the sender.

Ultimate come back used for any situation.
This come back was created by the great Bryan who we believe is a border line genius.