Penny: Honey, she's upset. You're her boyfriend. You have to at least try to be excited by the things she's excited by.
Sheldon: What if they simply don't excite me?
Penny: Well, just smile and think about koalas.
Sheldon: She'd see right through that. We go to the zoo all the time. She knows my koala face. And for future reference, it's this.