Materials science has a new concept to grapple with: mechanical memory. A team at ETH Zurich has engineered a metamaterial lattice — a carefully designed array of bistable unit cells — that accumulates a mechanical history. Under repeated loading, the lattice selectively flips specific cells into alternative stable configurations, changing the bulk mechanical properties of the structure in a way that is tuned to the loading pattern it has experienced. No electronics, no feedback loops, no external control. The memory is purely geometric and purely mechanical.