What point is goldman trying to make with his baseball analogy at the beginning of his talk?

Weight management is one benefit of participating in sports which sentance is true regarding weight management

As you sit in the library with your friend studying for your weekly quiz on the integumentary system, you come upon an article about the hazards of prolonged exposure to the sun's rays. your curious nature leads you to research the subject further. you're fascinated to learn that minimal exposure to the sun allows you skin to manufacture vitamin d, which is important to maintain homeostasis. you also read that although sunscreen can protect you from the ultraviolet (uv) rays of the sun, some medications can actually increase a person's sensitivity to the sunlight.

What is the only part of your body called that should touch the ball when setting in volleyball?