I wondered whether to approach the girl in front of me, especially as her ass was looking very inviting, but as I was thinking that, I felt someone move behind me and I felt a faint touch against my ass. I stood still -- it might have been an accident, but soon it came again... and again. First it was with the motion of the bus, so that maybe the person could be excused if I took offence, but then gradually, it got bolder and I felt the person leaning into me. I moved my ass back slightly, to let him know it was okay. .....