"Really? I was just kiddin' around - You can move boulders with your mind? That's incredible!"

Sure, he could do all manner of things with boulders. But they all involved his muscles. Not his noggin.

"Someday, we should go into the mountains together and train. We could have so much fun with that!"

He was imagining her hurling rocks at him with her mental powers, which he would then have to break. Maybe with one hand bound behind his back. Or maybe both.