system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does transparent materials like water act like mirrors?

When you look at a lake, or a water puddle, it reflects everything just like a mirror. How does it do this, if the water is totally see-through?
gpt
When light hits a boundary between one transparent medium (e.g., air) and another (e.g., glass or water), some is reflected and some is refracted.  Usually only a small percentage is reflected, except at very shallow angles, but often even that small amount is enough to produce a visible reflected image.  A good example is looking out a window at night, when it's dark outside but brightly lit inside.  The window will appear to act as a good mirror.  If you look out the same window in the day time the reflection is still there but it's much harder to notice because it's competing with the very bright view outside.