In practice the divisions are untidy. Even apparently similar materials such as different types of grain from the same family behave differently. Other materials veer between states. There are the slurries and heavy oils which though handled as liquids, are nearly solid under some conditions and water-borne solids, behaving as liquids when in transit and reverting to solids when drained. These are typified by pumped sand or minerals, water borne to transport them in bulk. Yet other substances gradually segregate, possibly into layers of varying constituency or where the characteristics of the original mix may have been imperfectly known from the start (11). As many containers have a very long life compared to that of their contents, there is the added uncertainty regarding changes in stored materials over time.