You must've heard of that. You've just forgotten. It's a fairy tale.
6 seconds sound clip from the Miracle on 34th Street (1947) movie soundboard.
You can hear this line at 00:09:22 in the Blu-ray version of the movie.
Quote context
[...]
- Mmm! He certainly is a giant, isn't he?
- Not really. There are no giants.
- Well, maybe not now, but in olden days, there were a lot of...
- Well, what about the giant that Jack killed?
- Jack, uh Jack. Jack and the Beanstalk.
- You must've heard of that. You've just forgotten. It's a fairy tale.
- Oh. One of those. I don't know any fairy tales.
- Oh, your mother and father must have told you a fairy tale.
- No. My mother thinks they're silly.
- I don't know whether my father thinks they're silly or not. I never met my father.
- You see, my father and mother were divorced when I was a baby.
- Well, that baseball player certainly looks like a giant to me.
[...]