SPEECH ACT THEORY in The Big Bang Theory

Once understood what a speaker can do (and most of the time does unknowingly) with language and how this is reflected in utterances, we may understand better the main character Sheldon Cooper and why he is the funniest one of the sitcom.

In other words, Sheldon does not perceive these uses people make of language, and thus, is utterly unable to perceive irony, sarcasm, etc. when somebody is messing with him or, on the contrary, when his friends do actually mean what they are saying.

Here you have a video which illustrates his "social/linguistic clumsiness":

Obviously, there are more factors (tone, gestures, facial expression) besides only the words Penny uses which hint that she does not want to be a waitress at the Cheescake Factory for the rest of her life.

It goes even further when Leonard asks somewhat rhetorically:
"For God's sake, Sheldon. Do I have to pull out a sarcasm sign everytime I open my mouth?"
As usual, Sheldon doesn't get the point and asks -more interested in the apparent fact (according to Leonards previous utterance): "You have a sarcasm sign?!".

The latter question is actually about another interesting topic: Presupposition.