A picture of Sheldon and Leonard's mother (Christine Baranski) looking over some brain scans of Sheldon's. The two of them get along frighteningly well.
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?