Which number is missing?

A five year old boy comes to visit his grandparents and notices his grandfather sitting on the porch, in the rocker, wearing only a shirt, naked from the waist down. "Grandpa, whatcha' doing? You're weenie's out and everybody can see!" he exclaimed.
Grandpa looked off in the distance, not answering.
"Grandpa, whatcha' doin' sitting out here with no pants on?" he asked again.
Grandpa looked at him and said, "Son, last week I sat here with no shirt on, Just watching the cars go by.... and I got a stiff neck. This is your Grandma's idea."