The Big Bang Theory2007

Penny:
This is between you and me. You can't tell Leonard any of this.

Sheldon Cooper:
You're asking me to keep a secret?

Penny:
Yeah.

Sheldon Cooper:
Well, I am sorry, but you would have had to have expressed that desire before revealing the secret, so that I could choose whether I wanted to accept the covenant of secret-keeping. You can't impose a secret on an ex-post-facto basis.

Penny:
What?

Sheldon Cooper:
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] 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.