One of my favorite subjects to paint and photograph is water. Why? Mostly because it is fluid and reflective. The water molecule is made of 2 hydrogen and 1 oxygen atoms. When they are bound together they form a very flat surface with the oxygen and hydrogen alternating in the same direction. This is great for causing photons (light particle) to bounce off in a consistent direction. They bounce because water is denser than air. Water is much flatter and smoother than most surfaces. You see reflections in water but not, say, sand, for the same reason you see your reflection in a polished piece of steel but not a rough-sanded piece of steel. All materials reflect light to some extent, but a rough surface scatters the reflected rays in all directions, so reflected images are blurred beyond recognition. On the other hand, with a very smooth surface, all the reflected light rays stay arranged in the same way they were arranged before hitting the surface, (except for being flipped into a mirror image, of course).
Last fall I was living in Venice, Italy, the magical city built upon water. Some of my favorite photographs are of reflections. With the slightest movement on the water, the images shift and distort—a portal to another world.