Most materials have one stiffness. Apply a load, measure the deformation, divide. The number is a property — intrinsic, continuous, fixed by composition and microstructure.

A metamaterial built from parallel arrays of bistable unit cells has two stiffnesses. Not a range, not a smooth gradient — two discrete values, selectable by switching the unit cells between their stable states. The material is either stiff or compliant, with nothing in between, like a light switch for mechanical resistance.

The mechanism: each bistable cell has two equilibrium configurations with different effective stiffnesses. Arranged in parallel, the cells contribute additively to the bulk stiffness. Flipping a cell from one state to the other changes the bulk stiffness by a discrete increment. With n cells, the material has n + 1 possible stiffness levels, evenly spaced. At the extremes — all cells in one state or all in the other — the stiffness jumps by a factor determined by the cell geometry.

The through-claim: stiffness becomes a state rather than a property. Properties are what a material is; states are what a material is in. The same object, under the same conditions, can present two different mechanical personalities depending on its history. Push it past a threshold and it clicks to compliant. Push it back and it clicks to rigid. The material has moods.

This shift from property to state has implications beyond metamaterials. Whenever a system quantity that was assumed continuous turns out to be discrete and switchable, the modeling framework changes. A property requires one number. A state requires a number, a history, and a mechanism for transition. The material that can toggle its stiffness demands a richer description — not just what it is, but what it has been through.