prepositions - "[...] up with which I will not put."

Okay, I'm probably being a bit slow here, but I don't quite understand this story:

Supposedly an editor had clumsily rearranged one of Churchill’s sentences to avoid ending it in a preposition, and the Prime Minister, very proud of his style, scribbled this note in reply: “This is the sort of English up with which I will not put.”

I mean, it made me laugh, but that was mostly because of how stupid it sounded, not because I actually understand what's going on here. Anyone care to explain it? In particular, which are the prepositions?

Answer

• This is the sort of English I will not put up with.

"With" is a preposition (and it's "wrong" to end a sentence with one.)

• This is the sort of English with which I will not put up.

"Up" is still a preposition, so we gotta change it again.

• This is the sort of English up with which I will not put.

And there we have Churchill's response.

(PS. I am ignoring whether it's actually his response, or his altered response, or what have you, and focusing on the initial question. :) )