It is kind of weird, when you think about it.
Today in the post office, I was approached by an Asian couple (I think Korean) in their or 40's or so. The woman asked a question - how were they to seal their envelope?
"Oh, you have to lick it," I said. I think they heard me, but thought they must have gotten the words wrong. "With your tongue," I elaborated, pantomiming the motion.
They were quite amused.
How many aspects of our culture do we learn by the age of six, never to think of again unless reminded that they are but aspects of our culture?