Your good friend punches you on the arm. How do you react?

• It doesn't hurt very much but the force is enough to move your shoulders a little bit
  
• You two are very good friends
  
• It was a completely random punch
  
• You are in another group of friends with him
  

This is related to Seddit in the different methods pickup artists give to responding to serious moments. I would like to know how some of you would handle it.