«What makes you think I’m 30?»

BILLIE: Whadaya got against babies all of a sudden?
PAUL: Nothing. I’ve got nothing against a brain that’s three weeks old and empty. But, after it hangs around for 30 years and hasn’t absorbed anything, I begin to wonder about it.
BILLIE: What makes you think I’m 30? I’m 29!

William Holden (as Paul Verrall) trying to interest Judy Holliday (as Billie Dawn) in education (George Cukor, Born Yesterday, 1950)