SA: Why is it then that, for example, my dog's fur is three inches long and it never seems to grow longer, while my own hair keeps growing and growing?
NS: Actually, a lot of types of human hair won't keep growing and growing. The normal length of the hair is an individual and species specific trait. So across the breadth of mammals, there are many norms for hair length, or fur length....
SA: Is a whisker a special kind of hair?
NS: Yes, it is. There are many different kinds of modified hairs to which we give different names. A porcupines quills are greatly enlarged hairs. Whiskers are hairs that work as sensory receptors. There's a strange animal from the Old World called a pangolin, which has these scaly plates that cover most of its body those are modified hairs.
By Valerius Tygart - Own work, CC BY-SA 3.0.