In xxxxAD, after a long voyage, Explorer-XXXIX discovered a new planet. Upon landing, its crew was fascinated by the golden ambiance of this planet. They promptly named it Aurumorbis, Gold Planet. Then, they radioed their finding to the Space Explorer Headquarters located on Earth, 'We've found Gold!'
After making some survey, one of them was puzzled by the ocean, 'Apart from the clouds and the sunlight, this place is a huge blob of ocean of gold. I have a bad feeling about this.'
Unbeknownst to the explorers, the ocean was slowly seeping into the spaceship. Or, to put it in another way, the spaceship was slowly becoming indistinguishable from the ocean.
Another explorer opened his food pack. He was startled by its contents. Instead of finding ration packets, there were gold bullions. Another explorer turned on the hydration unit. Instead of water dripping out, there were gold marbles.
You give the program a series of values corresponding to position and dynamic of the sun, and also that of the clouds, including how bright they appear, what their opacity is etc. Then the program takes these values and renders the result.
Of course, it's far more detailed than that, but I tend not to get involved with the semantics of the thing, just admire the results.