harlie'smom said:
What if the hair goes up on there back, what does that mean?
It's called Piloerection (hair standing up

) and basically it means they have reached a high level of arousal, the trigger for the arousal state may be fear, anger, pain, or excitement. It is an autonervous system response to stimuli. or as dictionary.com defines it: "involuntary erection or bristling of hairs due to a sympathetic reflex usually triggered by cold, shock, or fright or due to a sympathomimetic agent"
So it could mean that they are afraid of something, getting too excited in play, about to start a fight...etc. To tell the difference, you have to look at the overall body language. What postion are the ears? what does their muzzle look like? where are they putting their body weight? etc.
Now, my girl gets a "mohawk" when playing with my male or with a foster. To me this is a signal that things are getting out of hand or may get out of hand quickly. So I call a quick "training time-out" ask everyone to come to me, sit/down/shake, then give each a pat and send them back to play. Breaks the arousal chain and keeps things from escalating quickly. But that is my choice, made mainly for the safety and my sanity when fostering