Seth loves to arch his back and look behind him. I'm pretty sure that if he tried he could touch his head to his bum. He has always done this. Its really scary/annoying when you are holding him and he dives his head back. We always had to warn people that wanted to hold him because if you weren't prepared for it you could easily drop him.
He has learned that he can hang from things now. He loves to hold on to the bar under his high chair and hang out there. This is the only picture I have of him doing it and it isn't good, but you can see him hanging out.