Making Rapunzel’s hair look real was an enormous technological challenge. Byron Howard, who directed the movie with Nathan Greno, said technicians informed them early on that it would be best if Rapunzel’s hair never got wet, never got touched by another character and, if at all possible, didn’t even brush against her shoulders. “Hello!” he said. “That’s our entire movie.”

http://www.nytimes.com/2010/11/21/movies/21tangled.html

Except, you know, for all of the new programs they wrote to make her hair look more fluid and less plastic.