Evangeline and Rachel just saw the Star Wars trilogy for the first time last week. When we reached the famous scene when Darth Vader tells Luke that he is Luke's father, Evangeline burst into laughter. She suddenly understood the joke in Toy Story 2 when Zurg and Buzz are facing off on the elevator.
In some ways, I feel my daughters have missed something by seeing Star Wars at their age, or perhaps even in seeing it in the context where they know the joke first and foremost. There is no way they can appreciate what a shock it was to viewers who saw that movie for the first time and heard Darth drop that particular bombshell. I remember that even back then it was primetime grounds for debate and discussion. We had back-and-forth discussion for years about whether it was true, with people pulling out one bit of evidence or another to support their position. For my girls, there is no way the mystery ever could reach that level of suspense.
I rather feel they will have the same problem with "Harry Potter and the Deathly Hallows." Too much of the ending will be known to them, because so many of their friends already will have read it.