What is that back part of a ugg boot made out of? ?

I have been wondering 4 a long time what the back lil piece of a UGG boot in made out of....it seems like its made out of somthing diffrent the the othere kind because UGGs never lean and that part never gets bent out of shape...I want 2 say its made out of somthing like card bored but somthing a lil better