The joke occurs when they are in the restaurant and your heard something like "Yeah, but this one's eating my popcorn!". I'd heard that a long time ago but never bothered to look it up and unfortuntately the commentary didn't explain it so I searched the net and here it is!
 

Quote:An old farmer went to town to see a movie. 
The ticket agent asked, "Sir, what's that thing on your shoulder?"
 
The old farmer said, "That's my pet rooster, Rudy. Wherever I go, Rudy goes."
 
"I'm sorry sir," said the ticket agent. "We can't allow animals in the theatre."
 
The old farmer went around the corner and stuffed the bird down his pants.
 
He returned to the booth, bought a ticket and entered the theatre.
 
He sat down next to two old widows named Mildred and Marge.
 
The movie started and the rooster began to squirm.
The old farmer unzipped his pants so Rudy could stick his head out and watch the movie.
 
"Marge," whispered Mildred.
 
"What?" said Marge.
 
"I think the guy next to me is a pervert."
 
"What makes you think so?" asked Marge.
 
"He unzipped his pants, and his thing is out," whispered Mildred.
 
"Well, don't worry about it," said Marge. "At our age we've seen 'em all."
 
"That's what I thought, too," said Mildred, "but this one's eating my popcorn!"