Abstract: A beam for use in a lattice of beams of a mechanical neural-network (MNN) which can form a metamaterial configured to learn a set of properties by exposure to continuous external loads. Each beam of the lattice is configured to change its stiffness in response to external load input. A lattice of the MNNs is configured to learn by tuning the stiffness of its constituent beams in the MNN. The lattice can be either two or three dimensional, and configured to occupy volumes of arbitrary shape. The MNN has multiple layers of nodes, wherein each said node is analogous to the neurons within an artificial neural network.