Should I Hit My Son Back?

When is it okay to hit your child?  When the child hits someone else?  Today my 14 year old got suspended for hitting someone who pushed him.  It is never okay to put your hands on someone else, but is there a such thing as a reaction?  The someone who pushed him was a girl so now he is not just someone who hit a kid, but someone who hit a girl.  How do you punish your kid?  What would make him not hit a girl again, or even react with thinking first?  I am going to go the round about way and punish him severely without hitting him.  That might teach him that when you get mad you go ahead take it out on someone else.  Now, don't get me wrong, I believe in an old fashioned beat down every now and then.  But I think I'll wait for that one.