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The cigarette(or match) in the cup of gas trick works because the mix is too rich to burn, once the fumes mix with air then woosh up it goes!
I worked a house fire once where the guy was fixing a hole in his gas tank, he did everything right at first, drained the gas tank, filled gas tank with soapy water! Proceeded to spot weld gas tank hole closed, Here's his mistake, he drained the gas into a 5 gal bucket and set it to the side in the garage, he set it within 2ft of the gas hot water heater, fumes built up in the bucket overflowed to the pilot light in the water heater and away it went!