During preflight, one of the things you do is drain fuel from all the sump points, and make sure it smells flamable and looks blue. (100 low-lead aviation fuel is dyed blue). Why?
Condensation. Water vapor is in the air around us. The amount of water that can be held depends on the temperature. Once the air chills below that point, the water will condense, turn to fog/cloud, or in some other fashion re-appear into a for we know it as.
Unfortunately, fuel tanks, if not kept topped off, can have a fair amount of air in them. The plane I had today, had 13 out of 40 gallons filled, the rest of the space was air.
I did not find water in the wing drain points (where fuel is stored), but I did get water at the engine fuel drain. Not a huge amount, but some. It looked like a bubble that sinked to the bottom of the fuel sample. I drained and sampled repeatedly until all the water was out, and then I drained a bit more, to be certain.
The plane did run rough for the first moments, but cleared up quickly. More importantly, I didn’t feed the water into the system on takeoff. Otherwise, I might have emulated this accident in Rancho Murietta. There was a writeup in one of the flight magazines recently, by the instructor who was involved in that accident.
So, for today, I got to see a real life example of what you preflight so dillegently, and, I had ten touch-n-gos that didn’t suck. All in all, an interesting but good day.