Thesaurus

I remember the first time I used a Thesaurus. It was in sixth grade that I was introduced to one, and I was so excited about making my sentences sound intellectual that I would choose the wrong synonym. When I got my papers back, written on them -- in red ink -- would be something like, "This word doesn't fit with this sentence." Fortunately, I've learned since to cross reference words I'm not sure about with its definition from a dictionary. Why didn't they just tell me to do that in the first place? Or maybe they did...