When Penny, the waitress, smiles at her customers, she gets better tips. Here is a model: "Smiling makes customers think that their service is...

When Penny, the waitress, smiles at her customers, she gets better tips. Here is a model: "Smiling makes customers think that their service is better, so they give Penny more money." Why is this a bad model? Specifically address which of the three rules of thumb for model building this model has the biggest problem with. Then explain.