Students never fail to fascinate me when they ask WHY instead of HOW. Here is an example that happens today. (I crafted it into a question and answer session)
Student: How does light behave when it passes from air to a glass block? Teacher: The light ray bends according to Snell's Law. Student: How does the speed of light change as it passes from air to a glass block? Teacher: The speed of light decreases. Student: Why does the speed of light decrease?
Teacher: The molecules in a denser medium will cause the light to slow down. Student: Is energy lost in this process? Teacher: You can say that. Student: But how come the speed of light goes back to normal when it leaves the block? Teacher: *~*