Copyright and Fair Use LR

Question 14 surprised me first because I thought that as long as the audience was not paying to view the movie and secondly because I have done that. Many times I watched children for a function and the kids watched movies while the adults were in a meeting. I guess I was violating the copyright law without realizing it. I wonder though if I bought the movie and then showed it, would that be against the law? It would not be if the children were at my house, is it because the setting is in public? Or is it because the school/organization bought the movie? What if the teacher was throwing an end of the unit party and showed a popular movie that had nothing to do with what was studied? Or the teacher showed the Grinch at Christmas? Would that be against the copyright law because it was purely for entertainment and not for educational purposes? I guess this question opened up Pandora’s Box of questions.