The concept of "negative elephants," for example, makes no sense. Occasionally, however, it is possible to place real world meaning on a stock's "unreal" negative values. A firm's "negative inventory," for example, can be interpreted as its backlogged orders. From a system dynamics point of view, however, it would be far superior to model inventory and backlog as separate variables, rather than to use a single stock of inventory that can take on positive or negative values.