It begins early, the idea that there is only one right answer to a problem.
Here's part of a journal entry from when one of our children was in first grade:
She brought home several papers of the kind in which she had to identify beginning and ending sounds. The focus of one was a set of images, for which she was supposed to indicate whether the "p" sound came at the beginning or the end.
Next to the picture of a policeman, she had indicated that the "p" was at the end, and the the teacher had corrected it to the beginning, without further comment.
You can probably guess what comes next.
I asked our daughter what the picture was, and she replied, "cop."
What if I had not been there to assure her that her answer was perfectly correct, and to explain why the teacher thought it was wrong?
I spend a fair amount of time in my classes trying to assure students that there can be more than one right way to do a problem.