I've heard this story a few times over the years. I always thought the punchline meant something else entirely (something not so funny) until I finally got it (I think) yesterday. (Might be the way he tells it here.) Hilarious and offensive, be forewarned.

So I'm walking past the flower shop at the end of Rehov HaNasi today and pass an elderly nun walking in the same direction. At that moment, one of the flower shop employees comes out holding some plants. They clearly know each other.

"Shalom! Chag sameach!" calls the nun.

"Chag sameach!" replies the flower woman with a smile. "Mah shlomeich?"

"Toda la-El, toda la-El," the nun says happily. "Chag sameach."

Chag sameach to all!

(I think getting used to the liberal use here of the word "Elohim" is one of the things that will take the most adjustment, language-wise. Oh, and as it happens, in the novel we're reading in ulpan now, we've just gotten to the point where the main character meets a Hebrew-speaking nun. Go figure. )