The Truth of "Old Sayings"

Remember the old saying that doctors make the worst patients? Or how about a lawyer who represents himself has a fool for a client? A car repairman who fixes his own car gets it right the first time? A psychiatrist who analyzes himself is always schizophrenic? I don't know how much truth there is in these old sayings, but I can say unequivocally after having spent the whole day in a classroom with a bunch of teachers in a training session that TEACHERS MAKE LOUSY STUDENTS! or maybe THOSE WHO CAN DO, DO and THOSE WHO CAN'T DO, TEACH!