There are heaven-sent situations where you can teach by an example. It is like having life at your command, for a moment. One such case is when you get the learner to do -without peril (at least for them) - exactly the mistake you instruct him to beware. This is luxury education; once tasted in this way, wisdom is very difficult to forget.
This reminds me of a story:
The ageing Tamerlane sent after his favourite jester Nasreddin to come and tease his wits with some more words of amazing truth.
"Tell me Hodja, by your white beard", he said, "what else do I need to be remembered as a great ruler?"
"You have all the gifts in the universe but only need a little more patience, O Lord of the Fortunate Conjunction of the Planets," replied Hoca with a respectful nod.
"I see," said the Emir, "and what else do I need?"
"To always keep your calm and composure, Serene master," continued Nasreddin.
"So you say!" said Timur, "but what else?"
"Never to grow tired to listen, O, ear of the one God! Foe people are vain and boring"
"Is that all you can say?"
"More than everything, you need to be patient, my King."
"This is enough!"
"Patience and an even temper will lay the world at your feet, Incomparable one! In time, everything will end up as you wish..."
The Emir grew red in the face and shouted:
"You mock me, worm? How many times will you repeat the same dumb thing? Do you think I am too stupid to understand? Guards! Seize him! Let's see what else he has in his head!"
"Do you see what I mean?" whispered Hoca in Timur's ear as the guards approached, "I only repeated a simple piece of good advice a couple of times and already you lost patience."