I want to know why Cinderella's left behind glass slipper stayed glass and didn't change when everything else did. I mean, someone said it was cuz it wasn't touching her, but the pumpkin changed, ...

It's because otherwise there would be no stoty. :P

well, i don't know what a "stoty" is, but for the whole Cinderella thing, you apply this thing i call "suspension of disbelief" - it works every time