I found that my students couldn't push their works further because they were so afraid of ruining them, so I asked them to throw their works on the floor and rebuild them from the crushed forms. 

Later I aware I had a tendency to treasure my saved trash because I needed it for making works, and I was afraid the trash would be ruined.....Eventually, I remembered my instruction to my students and I started to squeeze and crush the trash before using it as a mold.