Penny: Right, okay look, this is between you and me, you cannot tell Leonard any of this.
Sheldon: You’re asking me to keep a secret?
Sheldon: Well I’m sorry, but you would have had to express that desire before revealing the secret, so that I could choose whether or not I wanted to accept the covenant of secret keeping. You can’t impose a secret on an ex post facto basis.
Sheldon: Secret keeping is a complicated endeavour. One has to be concerned not only about what one says, but about facial expression, autonomic reflexes, when I try to deceive, I myself have more nervous tics than a Lyme disease research facility. (Long pause.) It’s a joke. It relies on the homonymic relationship between tick the blood-sucking arachnid, and tic the involuntary muscular contraction. I made it up myself.
Penny: Okay, look, if Leonard finds out that I lied, I will absolutely die of embarrassment.
Sheldon: Physiologically impossible.
Penny: Oh Sheldon, please, look, I’m asking you as a friend.
Sheldon: So you’re saying that friendship contains within it an inherent obligation to maintain confidences?
Penny: Well, yeah.
Sheldon: Interesting. See, one more question, and perhaps I should have led with this, when did we become friends?