I think the reason is he did think before he threw the rock. His thought just wasn't about throwing the rock. Had he thought about throwing the rock, he would have but since he didn't think of throwing the rock until he thought of throwing the rock, he didn't throw the rock.
Thank you Cosmic! :) :) I must say, the thought of throwing a rock has only crossed my mind for skipping across the water or for decoration. I've missed you. It seems like weeks since you've been around. It's great to have you back. :) :)