The HOWS and WHYS ...

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: *~*

Food for thought.