Why does my gauge always tell me that my engine is running hot?

What is your question?

I have put anti-freeze in my 2000 Chevy Impala twice within the past two weeks. If I am not moving (even if I'm just temporarily at a stop at a traffic light), the hand on the meter starts swinging around to the hot side. Any time I'm not stopped, though, the hand slowly but surely starts to swing by over to the cool side on the meter. Anybody know why?