Sheldon: I am uncomfortable having been included in your lie to Penny.
Leonard: What was I supposed to say?
Sheldon: You could have told her the truth.
Leonard: I could not have said that, it would have hurt her feelings.
Sheldon: Is that a relevant factor?
Sheldon: Then I suppose you could have agreed to go.
Leonard: What would I have said afterwards?
Sheldon: I would suggest something along the lines of "singing is neither an appropriate vocation nor avocation for you and if you disagree, I recommend you do a CAT scan to locate the tumor which is pressing on the cognitive processing center of your brain".