Take a look at the image 1. Armando thinks that it is the very steep hill that makes this river narrow and deep, but his classmate thinks it is the small size of the sand grains that the water flows through. What information can the model provide us that would be used as evidence that Armando's idea is correct?

2. One limitation of Armando’s model is that all the water enters the model from one source: the water tank. Explain why this is a limitation of the model.