  Petri nets are an established graphical formalism for modeling and analyzing
the behavior of systems. An important consideration of the value of Petri nets
is their use in describing both the syntax and semantics of modeling
formalisms. Describing a modeling notation in terms of a formal technique such
as Petri nets provides a way to minimize ambiguity. Accordingly, it is
imperative to develop a deep and diverse understanding of Petri nets. This
paper is directed toward a new, but preliminary, exploration of the semantics
of such an important tool. Specifically, the concern in this paper is with the
semantics of Petri nets interpreted in a modeling language based on the notion
of machines of things that flow. The semantics of several Petri net diagrams
are analyzed in terms of flow of things. The results point to the viability of
the approach for exploring the underlying assumptions of Petri nets.
