Billy is four years old. He wanted to paint a picture at the easel but there was no paper left. He tried to ask the child development professional for more paper but she was busy with a group of other children. He looked around the room and saw some paper, which was being stored temporarily on a shelf above his head. He pulled up a chair beside the shelf, climbed on it, and reached for the paper. He took the bottom piece in both hands and pulled as hard as he could. The whole stack of paper toppled down on him. The teacher quickly ran to his rescue and yelled, "What are you doing touching the paper on this shelf