Half a joke: The Salesman's Robot

The Salesman's Robot

John was a salesman's delight when it came to any kind of
unusual gimmick. His wife Marsha had long ago given up
trying to get him to change. One day John came home with
another one of his unusual purchases. It was a robot that
John claimed was actually a lie detector.

It was about 5:30 that afternoon when Tommy, their
11-year-old son, returned home from school. Tommy was over
two hours late.

"Where have you been? Why are you over two hours late
getting home?" asked John.

"Several of us went to the library to work on an extra
credit project," said Tommy. The robot then walked around
the table and slapped Tommy, knocking him completely out of
his chair.

"Son," said John, "this robot is a lie detector. Now tell us
where you really were after school."

"We went to Bobby's house and watched a movie," said Tommy.

"What did you watch?" asked Marsha.

"The Ten Commandments," answered Tommy. The robot went
around to Tommy and slapped him again, knocking him off his
chair once more. With his lip quivering, Tommy got up from
the floor, sat down, and said, "I am sorry I lied. We really
watched an R-rated movie."

"I am ashamed of you, son," said John. "When I was your age,
I never lied to my parents." The robot then walked around to
John and delivered a whack that nearly knocked him out of
his chair.

Marsha doubled over in laughter, almost in tears, and said,
"Boy, did you ever ask for that one! You can't be too mad
with Tommy. After all, he is ---...

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I omitted the ending of the joke! Wanna guess the ending? Good luck!