How did this happen?

[bw4sale]

I finally got to go to my Dad's for the day and work on my truck a little. While I was trying to get it started (been a while), I got out and went around to the front and just for a second (I don't know why) I touched the postive post on the battery. Here's what I don't believe happened except that I was there and it did. When I touched the post, I felt the slightest shock and for a fraction of a second the starter turned over. Switch was OFF. What happened? Like I said, I wouldn't believe it if I heard it. In fact, I'm starting to doubt my self. Must be my electric personality.