inception

just watched it and it was great. I have one question, what was the seed that he planted in mal's mind during their stay in limbo, i know it is the spinning top, but what about that made her realize that it was fake, and what made him realize that it was fake?

what most people justifications are is that he shows her the top not being able to stop spinning in limbo, but when they are back in the real world he could just show her it spin and stop and then she should believe.