system
Given the following post, respond as if you're commenting on Reddit:
human
Why does the shadow of a bottle of water show up as a "dark" shadow even though water is clear, and the bottle is clear.

Context:

I was holding a bottle of water and looked down at the shadow of it and it was showing up as a "shadow" and not like I expected.  The shimmer was showing up on the ground, but the shadow was basically like if I had motor oil in the bottle.  Weird question, but it doesn't make sense to me.
gpt
A bottle of water is basically a sort of misshapen lens.  If you take a proper lens and try the same thing, you'll see it casts a shadow except where it's focusing light.  The light that would shine where the shadow is has been diverted to the bright spot.  The effect is just less obvious for the bottle, because it's focusing light in all kinds of crazy, constantly-shifting places.