"Stiff neck"

A man was walking down the street when he noticed his grandpa sitting on   
the porch, in the rocking chair, with nothing on from the waist down.   
 "Grandpa, what are you doing?"  he exclaimed.  The old man looked off in   
the distance and did not answer him.  "Grandpa, what are you doing   
sitting out here with no pants on?" he asked again.  The old man slyly   
looked at him and said, "Well last week I sat out here with no shirt on,   
and I got a stiff neck.  This was your Grandma's idea!!"