disneymoviesandfacts:

By the time Toy Story 3 was made, Pixar animators had figured out how to animate things like water and fur. Although being able to realistically animate fur was originally a concern for Toy Story 3 animators, the real animation challenge was trying to animate the trash bags in the movie. Because trash bags have special properties such as how it reflects light, animators spent weeks trying to get the trash bags correct.

disneymoviesandfacts:

By the time Toy Story 3 was made, Pixar animators had figured out how to animate things like water and fur. Although being able to realistically animate fur was originally a concern for Toy Story 3 animators, the real animation challenge was trying to animate the trash bags in the movie. Because trash bags have special properties such as how it reflects light, animators spent weeks trying to get the trash bags correct.