Our four year old has been saying "no" to everything.
In exasperation we finally said, "You, Mister Fister, are not allowed to say 'no' anymore."
"YES I CAN!" he yelled. (He was not having a good day.)
"Excellent! We like how you just said yes," we replied calmly.
To which he looked thoroughly confused and slightly insulted...as if we tricked him, or something.