A Lego set is justly famed for allowing you to build models with the property of extensibility. Lego also offers a clue as to how to achieve extensibility - go down to the level of a very basic component, and build more complex elements only from them.
Very few of our physical constructs are like that - you can't stick two airplanes together and hope to get something that works. Physical components normally need to be carefully crafted to fit and work together.
We can build computer-based systems that are extensible if we do the same thing as Lego - go down to some very basic element. The Active Structure paradigm goes down to the most basic elements - a PLUS operator, an EQUALS operator, allowing you to build extremely complex systems based on them, and yet being sure that extensibility of the system in any direction is still possible. The structure also needs the property of undirectedness to be extensible, just as a piece of Lego or Meccano is undirected to any particular purpose.
See Self-Extensible