Why is it necessary to have your car engine switched off before your fill up your car?

Why is it necessary to have your car engine switched off before your fill up your car?
The reason why you should turn your engine off is so you don't get exploded!

Gas fumes are heavier than air, falling to the ground at every fill up as the fuel pushes the vapors in the tank out the fill tube. Given the proximity of the catalytic converter to the ground and the purpose of the cat being to burn the so-called "Waste Fuel", you have a recipe for disaster.

You can ignite the fumes, the flame traveling at super-sonic speed to the gas tank and/or the pump.

You will also want to leave your cell phone in the car. Radio Waves act like microwaves, exciting the entire area electro-magnetically. Many have had their phones ring while at the pump, and wind up in the hospital with 3rd degree burns.

Taking that call, or leaving your engine running is a recipe for disaster. Ask yourself one question: "Is it worth the risk?"