“Be sure about the problem in the first place before working hard to solve one."

Funny story on Homework
A little girl came home from school and said to her mother, "Mommy, today in school I was punished for something that I didn't do."

The mother exclaimed, "But that's terrible! I'm going to have a talk with your teacher about this. By the way, what was it that you didn't do?"

The little girl replied, "My homework."