I know, I know, it can be caused by various factors, like high ISO, poor lighting conditions, long exposure time, heat. (not that I know why the last two cause noise)
But where does noise REALLY come from? What is noise? What does it have to do with photons and photosites?
It also comes from my wife when I whip out the DSLR at a particularly interesting location while on vacation and say “I’ll just be a minute…”