When I was a little kid, we were told not to pick our noses. And it was made quite clear that this didn't mean "don't pick your nose in public", but "don't pick your nose at all, even in private".
The funny thing was that nobody ever offered any viable alternatives to picking one's nose, or at least not any alternatives that didn't assume that the contents of the nose had to be liquid.
I am curious: did anyone's parents ever really say what to do instead of picking one's nose? Or did the parents in the civilized world just tell their kinds not to do it in public?