Pop is a bear from the book Hop on Pop.
He doesn't appreciate his two children hopping on his belly. When Pop's children hop on his belly and say, "We like to hop! We like to hop on top of Pop!", he is surprised at them hopping on him. Then he gets mad and annoyed, and angrily stops them. He says, "STOP!" (trying to stop them from hopping on him). Then he continues, "YOU MUST NOT HOP ON POP!".