SOMEONE HAS SAID That the abiding impulse in every human being is to seek order and harmony. This search for order is clearly visible in our study of nature. We see that many objects are rigid, but that others flow easily and do not have definite shapes. We want to distinguish these types and we even give them names-solids and liquids. When we try to describe substances such us tar, butter, and glass, which do not fit well into either the solid or the liquid class, we may be somewhat confused. We many decide arbitrarily to put them into one class or the other, or we may think of another class or a new subclass.
