- It's deep water, that's why a duck. It's deep water.
- That's why a duck.

3 seconds sound clip from the The Cocoanuts (1929) movie soundboard.

You can hear this line at 00:48:18.768 in the DVD version of the movie.

Quote context

[...]

- Well, we'll pass over that. You're a peach, boy.

- Now, here is a little peninsula, and here is a viaduct leading over to the mainland.

- Why a duck?

- I'm all right. How are you?

- I say here is a little peninsula, and here's a viaduct leading over to the mainland.

- All right. Why a duck?

- I'm not playing Ask Me Another. I say that's a viaduct.

- All right. Why a duck? Why a duck? Why a no chicken?

- I don't know why a no chicken. I'm a stranger here myself. All I know is that it's a viaduct. You try to cross over there a chicken, and you'll find out viaduct.

- It's deep water, that's why a duck. It's deep water.

- That's why a duck.

- Look, rube. Suppose you were out horseback riding, and you came to that stream and wanted to ford over. You couldn't make it. It's too deep.

- But what do you want with a Ford if you got a horse?

- Well, I'm sorry the matter ever came up.

- All I know is that it's a viaduct.

- Look. All right. I catch on to why a horse, why a chicken, why a this, why a that. I no catch on to why a duck.

- Well, I was only fooling. I was only fooling.

[...]