Bill had a small farm. He was doing well, but he had a problem with large flocks of crows on the farm. He decided to call in a specialist with a rifle to help him getting rid of them.
He found one advertising in the local phone book and called him. Things were arranged. He would come the next day and shoot the crows with his rifle. "What time will you be here?", Bill asked. "At 8 or 9 in the morning", he answered.
The next morning he arrived at the farm with his rifle at 8 in the morning. He was good. He shot a lot of crows, but did not manage to get them all, so they agreed he would come again the next day. "I will be here at 8 or 9", he said.
The next day he arrived at 8 in the morning and finished off the work. Then he sat down with Bill to have a cup of coffee. "You did a good job", Bill told him. "But I noticed that yesterday you put the rifle butt against your left shoulder, while today you put it against your right shoulder. How come?".
"Well, you see", he answered, "when I wake up in the morning I look at my wife sleeping next to me. If she is sleeping on her left shoulder, I put the butt to my left shoulder. If she is sleeping on her right shoulder, I put it to my right shoulder."
"What if she is sleeping on her back?", Bill wondered.
"If she is sleeping on her back I come at 9", he said.