Whilst teaching timetables in school one day. The teacher started by writing the nine times table on the board. She deliberately put the wrong answer for the first sum.
For the rest of the list she wrote the timetable correctly. When she was finished, she looked at the students and they were laughing at her because she had got the first sum wrong.
The teacher then explained to them that she had wrote that first sum wrong on purpose to teach them something important. She informed them she had deliberately done that to demonstrate how ruthless the world is. She pointed out to the students that although she had wrote the timetable correctly nine times, everybody had ignored that fact to focus on the one error she had made. No one congratulated her for getting nine correct answers. Instead they had all laughed and criticised her for the one sum she got wrong.