Figure 1.

(a) Input and output processes are necessary for any system to be an ‘open system’. (b) Turnover connecting input and output processes is necessary in any non-trivial open system in which what goes in does not simply come back out. (c) Storage, as a means of bringing materials on and off the turnover ‘production line’, is a second essential process in any non-trivial open system. Then useful materials can be used at a later time, or components can be combined in an order different from that in which they entered.