Throw a lit cigarette or match into a bucket of gas, it'll put the fire out. And not start the gas on fire. Strangest thing.
Unless fully cleaning the tank out with water, there will be gas vapors left, which WILL explode.
You are 100% correct about the cigarette thing. Gas just doesn't burn in a liquid state.
As for the water thing, you are correct as well. But its needs REALLY good cleaning. Gotta be careful. I was trying to repair a plastic ATV fuel tank once. I needed to heat up the threaded area where the fuel cap screwed onto, so I could re-form it to get the cap to seal. I removed the tank form the quad, drained the fuel, and rinsed it like 20 times. Put some water in, shake it around and drain, repeat, repeat, repeat, etc.
Put the torch up to the fuel tank cap area to heat the plastic and BOOM! Next thing I know I'm laying on my back on the garage floor wondering where that burn hair smell was coming from.
Filing the fuel tank all the way up with a liquid is the best way to go. Fuel, water, whatever. Just gotta get rid of those fumes.
If you take a metal fuel can, remove the cap, fill it all the way up with fuel and then light it - you don't have a bomb. You have a petroleum lamp. But with only a small amount of fuel in the bottom of the can - light it and you will smell burnt hair - if you are lucky.
I learned a lot about fuel and fire over the years. (profile pic was not chosen by accident).