I remember John Ramesh in class 6, who was very serious in securing first rank, but missed by a few marks that time. After the half yearly exams, he went to Mr.NCS (our class teacher) with his answer sheets. He had lost a couple of marks since he made a small mistake of a (-)ve sign. Ramesh pleaded "Sir, it's a very small mistake, after all you can see that I have done it correctly in the work space. But the mistake was done while writing the answer from work space, so you should consider it"
Mr.NCS smiled and told us a story in his typical style. 
"A train started from Pune to Bombay, The station master was happy (he is going to complete his duty & can go home), the engine driver was happy (he had only a few hours from completing the job), the passengers were happy (they will be at their destinations, may be with their longing families in the matter of a few hours). The train reached Bombay. The station master at Bombay was happy (the train reached safe), the driver was happy (completing the successful job), but the passengers were not happy. why ? why ?. A small mistake, the compartments were not connected to the engine." 

Mr.K.V.Rao was demonstrating the formation of magnetic lines by using a magnet and iron fillings.
He asked me go inside and bring a magnet and a bottle of iron fillings.
Naughty as I was, I picked up the magnet and a bottle of Potassium permanganate, which looked very similar to iron fillings.
Mr. K.V.Rao, in his typical style put the magnet on a big white paper and the iron fillings around it. And started tapping the table to form magnetic lines.
Just then our Princi Wg.Cdr.Das Gupta walked in on a surprise inspection and stood watching the demonstration.
Tap. tap. tap. nothing happened. This went on for about ten minutes. Nothing happened. Then Mr. K.V.Rao, quite embarrassed, straightened himself and said " this magnet is not working now, because there is a bigger magnet in the class, our Principal."
Princi walked out smiling and all the class had a good laugh, except me because I was thrashed by Mr. K.V.Rao.