Inland waters may be grouped into two general classes: standing waters and flowing waters. As is often the case, the boundary between these two classes is not sharp and clear. A pond is an example of standing water. But most ponds are fed by springs or brooks and most have an outlet. Thus some current of changing water flows through them. On the other hand, a river is an example of flowing water. In some places, however, a river may have such a slow current that it is very difficult to detect. Standing inland waters differ in size, in age, and in many _ environmental characteristics. They range in size from roadside puddles to the Caspian Sea. Puddles may last for only a few days or weeks; ponds, for a few hundred to a thousand years. In general, lakes are older, though the waters of some tropical "lakes" disappear completely during each dry season. Standing waters vary from very shallow to very deep, from clear to muddy, from fresh to salty. In flowing waters we roughly distinguish between brooks, creeks, and rivers. The size and age of flowing waters are unimportant. Speed of flow, clearness, oxygen content, and other chemical characteristics are used by scientists in studying flowing-water ecosystems. This passage mainly tells us that   _  .
there are two major classes of inland waters lakes and rivers are flowing waters age and size of flowing waters are important ponds and brooks are standing waters