Materials and being principled

Our class showed enthusiasm and strong principles last week as they combed the school ground looking for materials that do NOT belong in nature.  They were surprised by how much garbage was on the ground, and really surprised when we opened the garbage cans and saw that they were virtually empty.  We took care of cleaning up the school grounds, and felt really good about it afterwards, but they were left wondering why anyone would choose to litter, rather than put their garbage in the trash?