Star Trek: The Next Generation1987

[Worf and Guinan during target practice]

Guinan:
You know, I had a bet with the Captain that I could make you laugh before you became lieutenant commander.

Lieutenant Worf:
Not a good bet today.

Guinan:
I've seen you laugh. I like it.

Lieutenant Worf:
Klingons do not laugh.

Guinan:
Oh, yes, they do. Absolutely they do. You don't. But I've heard Klingon belly laughs that'd curl your hair.

[both continue with their target practice for a while]

Guinan:
Your son laughs. He's Klingon.

Lieutenant Worf:
He is a child and part Human!

Guinan:
That's right. And you're not; you're a full Klingon, except... you don't laugh.