Can Anyone easily explain how and why static builds up on someone in a car or truck?

I have wondered why I get a shock sometimes getting out of the vehicle. I never really thought much about it. Today after getting gas (self serve) I got back into the van and started to drive away, looked in the mirror and saw that I had left cap off and door open (mind was elsewhere I guess). I stopped and got out to put the cap on and got a nice shock when putting the cap on (touched metal somewhere). Having a spark in front of the entrance to my gastank really caught my attention.

Anyone know of a way to keep this from happening?


Bill