Word or Vocabulary usage

Yesterday, I worked with a child who was concerned about a test.  She had a test in science and had done poorly on the study guide.  The teacher had given the class the right answers to the ones they had missed.  Why then did this student insist she needed help?  First, I had her stand and work on the balance board so she could regroup and think in a different direction.  Then I looked at the question and asked her why it was hard for her to remember the answer.  The question asked what instrument had to be invented before the cell could be studied.  The answer was microscope.  She did not understand what instrument meant in the science context.  I explained that it was a tool like her uncle used to build something.  It was like a hammer and nails, or saw he used.  When we talked about how it meant a tool, she could understand and remember the answer. Several of the questions had words that she just had no understanding to build on.  How often do we forget what it is like to have a clue missing when we learned something?  As we were  studing for the test I had her stand on the balance board to repeat the answers once she understood the questions asked and correct answers.  She did not have the book so when I did not remember one word meaning, she was surprised when I called and had a friend google the word meaning.  I think one reason she will ask me questions is she sees me ask others.  She still said I could not say I did not know the answer.  I laughed and told her I did this all the time.
Have you ever known you should know something but just not been able to come up with the answer?  When we allow children to see our weaknesses, we allow them to learn without the embarrassment of not knowing.