Final answer:
Normal cells stop dividing when they reach confluence, while malignant cells continue to grow and divide even after reaching confluence.
Step-by-step explanation:
When comparing the growth and division rates between normal and malignant cells cultured under favorable conditions, an unexpected finding is that normal cells stop dividing when they reach confluence, which is a single layer of cells covering the culture dish. This phenomenon is called contact inhibition. In contrast, malignant cells do not stop dividing at confluence and instead continue to grow and divide, piling up in multiple layers. This difference in growth behavior between normal and malignant cells contributes to the uncontrolled cell growth seen in cancer.