Canine Thoughts

What thoughts go through a dog’s mind?  Our 8 year old mongrel seems to understand when our conversations are about her or an activity she enjoys. I don’t know if it’s our unconscious use of a different tone of voice or if she recognises certain word sounds that have a happy connotation for her but she can be lying in her bed, at peace with herself and the world and then, in the middle of a discussion, for no apparent reason, she will begin to wag her tail or get up and sit, looking expectantly at us.  I wonder why?