Why Does Water Reflect Images Like This?

When nearby lakes overflow during the rainy season, the salt flat will fill up with a very shallow sea of water. In some places it appears to be only inches deep. A ery still puddle like that can essentially create a gigantic mirror, reflecting the sky all the way to the horizon. And in some places, you can just walk on it like it's so shallow it looks like you are walking out over the mirror that goes on forever. So one thing i was wondering about is why exactly does water reflect images like this, while so many other materials don't?