Spring Mass Systems:  Structures from Springs

A wide variety of flexible, non-rigid structures can be built by hooking together small particles (each with some mass)
with simple springs (which may include some internal damping).

Examples of spring mass "sheets" in 2D:

Examples of spring-mass clusters, blocks, and trusses in 3D:

These structures can only resist folding in directions where there are springs to prevent or slow down such folding. Examples:

Such structures are useful to model a block of Jello or a sheet of cloth ...

Sometimes it is advantageous to control the angles in an articulated structure.
Then it makes sense to have springs that control the deviation of any joint angle from some rest angle.
Similarly for structures made from plates that are joined with hinges along their edges, we can use

This technique is particularly useful in dynamic articulated skeletons, e.g., "rag dolls".