We started with a clear glass full of water. We can see right through the water, because there are no particles in it. It's clear.
Then we added just a little water. Suddenly we can no longer see through the water. During the day we can't see the stars, because the sky is filled with particles that the sun is shining off of, creating a blue look (because it is a dominant color in our eye sight).
I personally learned a good deal during this mini lesson. It was lots of fun, and the Princess' asked if we could do it again.