Pretend a person can walk only at a certain pace - no faster, no slower. If you time her uninterrupted walk across a room of known length, you can calculate her walking speed. If, however, she stops momentarily along the way to greet others in the room, the extra time spent in her brief interactions gives an average speed across the room that is less than her walking speed. How is this like light passing through glass? In what way is it not?