When I was a kid growing up in the late '50s, the restaurants had a jar of mustard and a jar of mayonnaise at the table, and you used your table knife to spread

When I was a kid growing up in the late '50s, the restaurants had a jar of mustard and a jar of mayonnaise at the table, and you used your table knife to spread whichever you wanted on your sandwich, just like you would at home. This was a problem if you wanted both mayonnaise and mustard. It wouldn't be polite to contaminate one with the other, plus you might get bits of lettuce and bread in the jar. So we were taught that after putting the mustard on the sandwich, rather than dip the dirty knife in the mayonnaise jar, we should first lick it clean.