If you allow gasoline to fall through mid air, static electricity builds up, there is a spark, and pow!

There is a ground wire imbedded in the fuel hose at the gas station.

Any time that you allow gasoline to flow, there must be a good ground between the containers. Car-to-can must be grounded.

This is easier with metal containers. With a plastic container, you may need to actually put a wire in the path of the flow and make sure that the wire touches the car and the gasoline in the can.

I have seen several fires started when someone is draining a fuel tank into a container without a ground, including a 4-bay truck repair garage that was totally destroyed. Interestingly, it was a diesel garage, but an employee was working on his own gasoline fueled pickup truck!
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'96 855R,'64 PV544, '67 P1800, '95 855, '95 854, the first three are mine, heh, heh, 405,000 miles put on 8 bricks