As an aside, the original quote was from Aristotle, who said that 'Those that know, do. Those that understand, teach'. This was later parodied as 'Those that can, do. Those that can't, teach' and later even further as 'Those that can, do. Those that can't, teach. Those that can't teach, become guidance counsellors'. But few people realize that the latter two are both parodies of the original, which was intended to be complimentary.