It's really important for students to explore. When students get their first chance to paint, many spend the whole class period painting sheet after sheet. Usually after one or two classes of this (and the knowledge that they can use more than one sheet of paper!) they stop before the mud stage. This kindergartner found a good stopping point:

The kids were excited that I added some new treasures to the collage center:

"I just made this! It might be to give to my mom, but I might have to keep it. Can you see where I wrote on it?"