ON OFF Switching

A switching device is any mechanical, electrical, magnetic, pneumatic or hydraulic device designed to open or close an electrical circuit (permit or interrupt the flow of electrons in a circuit).

Switches are essentially binary devices: they are either completely on ("closed") or completely off ("open").

For power applications, when closed, switches are required to carry a certain amount of continuous current without overheating, and in the open position they must provide enough insulation to isolate the circuit electrically.
Device capabilities are represented by some general parameters like power (max current, max voltage) and max speed (switching frequency) and also control flow: unidirectional or bi-directional.
The power capability propriety can be enhanced using parallel pairing or Darlington pairing.

Switch examples:

• Electromagnetic Relay 
• BJT - Bipolar junction transistor
• MOSFET - Metal–oxide–semiconductor Field-effect transistor
• TRIAC - Triode for Alternating Current 
• SCR - Thyristor, Silicon-controlled rectifier
• Solid State Relays
• UJT - Unijunction transistor
• GTO - Gate Turn-Off thyristor
• MCT - Metal–oxide–semiconductor Controlled Thyristor
• JFET - Junction gate Field-effect transistor 
• IGBT - insulated-gate bipolar transistor
• H bridge (Full-H or Half-H) (solid-state or mechanical)

LEGO vs Puzzle

Lego bricks can be assembled and connected in many ways, anything constructed can then be taken apart again, and the pieces used to make other objects.

Lego pieces of all varieties constitute a universal system. Despite variation in the design and purpose of individual pieces over the years, each remains compatible in some way with existing pieces.

A puzzle is a problem or enigma that tests the ingenuity of the solver. In a basic puzzle, one is intended to put together pieces in a logical way in order to come up with the desired solution.

   So puzzle is a challenge with an expected solution while Lego is a creative game first imagine something then try to build it - the solution is not unique.

Abstract layers and modular design.

An abstraction layer (or abstraction level, or a layer of abstraction) is a way of hiding the implementation details of a particular set of functionality.

The simplification provided by a good abstraction layer allows for easy reuse.

Modularity is the degree to which a system's components may be separated and recombined.
wiki - Modular design
Modular design, or "modularity in design", is an approach that subdivides a system into smaller parts (modules) that can be independently created and then used in different systems to drive multiple functionalities. A modular system can be characterized by the following:

1. Functional partitioning into discrete scalable, reusable modules consisting of isolated, self-contained functional elements
2. Rigorous use of well-defined modular interfaces, including object-oriented descriptions of module functionality
3. Ease of change to achieve technology transparency and, to the extent possible, make use of industry standards for key interfaces.

Besides reduction in cost (due to lesser customization, and less learning time), and flexibility in design, modularity offers other benefits such as augmentation (adding new solution by merely plugging in a new module), and exclusion.

In industrial design, modularity refers to an engineering technique that builds larger systems by combining smaller subsystems.

Anything constructed can then be taken apart again, and the pieces used to make other objects.