One of the Best Movie Scenes Ever
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Here's a quote from Wikipedia about this scene:
The film may be best known for a scene featuring the two title characters having lunch at Katz's Deli in Manhattan. They are arguing about a man's ability to recognize when a woman is faking an orgasm. Sally claims men cannot tell the difference, and to prove her point, she vividly (but fully clothed) demonstrates the skill as other diners watch. The scene ends with Sally casually returning to her meal as a nearby patron, played by Reiner's mother, places her order: "I'll have what she's having."