Perhaps you have observed the following wear phenomena: A set of marble or wooden stairs shows wear on the treads in the regions where people step when they climb (or descend) the stairs. The higher up the stairs, the less the wear on the tread. Given that soles of shoes (leather, rubber) are far softer than marble or granite, explain:
a. Why and how the stairs wear.
b. Why the lower stairs are more worn than the upper stairs.