Big Bang Theory Quote
Penny: This is between you and me. You can't tell Leonard any of this.
Sheldon: You're asking me to keep a secret?
Penny: Yeah.
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.
Penny: What?
Sheldon: Secret-keeping is a complicated endeavor. One has to be concerned not only about what one says, but about facial expressions, autonomic reflexes. When I try to deceive, I myself have more nervous tics than a Lyme disease research facility.
(pause)
Sheldon: 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.
Quote from S02E01 - The Bad Fish Paradigm