Johnny Carson loved the silly metaphors:
Johnny: Boy is was hot today. (Pause)
Audience: How... hot...was... it?
Johnny: (Mugs for the camera, obviously enjoying the audience participation) It was so hot that a dog was chasing a rabbit and they were both walking.
So now David Letterman, a great fan of Johnny, copies the format.
David: Boy it was cold today (Pauses)
Audience:(Titters, knowing it is expected to participate, but may not..which is even funnier.)
David: It was so cold that the squirrels in Central Park were lighting fires in trash cans to warm their nuts.
Imitation is the sincerest form of flattery.