A polyhedron is rigid if it cannot be continuously deformed into another configuration. A rigid polyhedron may have two or more stable
forms which cannot be continuously deformed into each other without bending or tearing
A polyhedron that can change form from one stable configuration to another with only a slight transient nondestructive elastic stretch is called a multistable
polyhedron (Goldberg 1978).
A non-rigid polyhedron may be "shaky" (infinitesimally movable) or flexible. An
example of a concaveflexible polyhedron
with 18 triangular faces was given by Connelly (1978), and a flexible
polyhedron with only 14 triangular faces was subsequently found by Steffen (Mackenzie
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