All dogs are brilliant at perceiving the slightest movement that we make, and they assume that each tiny motion has meaning. So do we humans, if you think about it. Remember that minuscule turn of the head that caught your attention when you were dating? Think about how little someone's lips have to move to change a sweet smile into a smirk. How far does an eyebrow have to rise to change the message we read from the face it's on—a tenth of an inch? You'd think we would automatically generalize this common knowledge to our interactions with our dogs. But we don't.

— The Other End of the Leash by Patricia B. McConnell (Page 1)