Lesson on Seeing Light and Shadow

I'm currently teaching a classical drawing class to a group of home-school students ages 6 - 13!  It's been said that learning to draw is really learning to see.  We spent the first half of the first day looking at water drops.  Each student had a styrofoam plate with water drops on it.  I brought in a lamp from home so that there would be one direct source of light.

"Water is clear so how is it that we are able to see these drops?  Should they be invisible?  Do you see any thing except the white of the plate?  Which half is light?  Which half is dark?  Did you know that water can cast a shadow?  Do you see the slight glow hitting the plate in the middle of the shadow?  Is the shadow next to the dark part or the light part?  Do you see that tiny bright spot on the darker part of the drop?  Look very closely and you will actually see the shape of the light bulb reflected there (these photos were taken in the sunlight, so you won't see the lamp's shape!).  Do you think you could draw and water drop?  What colors might you use?"