What does Bert want to see in Joe's house?

Bert wants to see the jail.

Bert: {pulling him down by the lapel and whispering in his ear} Can I see the jail now?
Keller: Seein' the jail ain't allowed, Bert. You know that.
Bert: Aw, I betcha there isn't even a jail. I don't see any bars on the cellar windows.