What part of the body could you hit somebody and leave no marks?

Question

So, where could you hit somebody and there would be no marks left behind. Like if you hit somebody with the fist in the face, there will be a huge mark afterwards, so the police could tell that you (or somebody) did harm to him. So, where could you hit him so that he can't get any after-marks, so he can't go to the doctor or the police and say he has been hit...or they could not tell.

Thanks. I will choose a good answer so if you know your stuff, please post, you'll get 10 points.

Answer 1

Er, what a strange question! Why?

Answer 2

on the top of the head were there is hair