"Is it?" he asks, all innocence, and latches onto the spot that seems to be making Stiles move the most. It's pretty fantastic, the way Stiles is reacting to what in Derek's opinion is very little touching. He can feel how turned on Stiles is as he shifts against him, and Derek inhales at the feeling of Stiles's cock inside his jeans.
He nips at the mark he's just made, his hands sliding around to grip Stiles's ass, encouraging the movement.