What Motivates a Child to Read?

I left this sentence on the board (a window in this case) for my first-graders, then told them that it contained a secret that I would neither read nor teach. Their curiosity was piqued and for two days they snuck glances between lessons, writing secretly in their notebooks and teaming up on the big words. One child even implored that he be allowed to stay in during recess to work on it. Sorry, buster, you have to suffer out there like the rest of 'em. A few days later, after slowly piecing it together, they raced out to the propane tank and found the container of chocolate I had taped there.

I don't mean to suggest this was a perfect lesson plan, just that I'm curious - what motivates a child to read?