When Leonard tells Sheldon he needs to use more honey than vinegar on Penny, he tries an experiment where he uses chocolate as positive reinforcement on Penny. Amazing.
Sheldon: I'll have a diet Coke. Penny: Can you please order a cocktail? I need to practice mixing drinks. Sheldon: Fine... I'll have a virgin Cuba Libre. Penny: That's... rum and Coke without the rum. Sheldon: Yes, and would you make it diet?