The surface of many materials don't just reflect
the color of the material itself but also reflect the shapes from other
objects. Water is certainly the most common natural "mirror" that comes
to mind but polished metal or glass work just fine as well. Obviously mirrored
objects offer some interesting photographic opportunities.
An exact mirroring of the main subjects
is often not necessary. Especially with night scenes the contours of a
subject are lost but some rough reflections still offer a pretty neat impression.
The following picture just shows the reflections of the light source but
not the form of the sky scrapers.