We typically think of memories as occurring in magnetic storage devices or neural networks, but memory effects abound in a wide range of materials. Rubbers and rocks can remember the largest loading applied to them; glasses may remember their past relaxation; shape-memory alloys remember a programmed shape. Many more examples exist in physics, biology, and chemistry. Despite the ubiquity of memory formation in condensed matter, there is presently no overarching framework for categorizing and describing such material memories. I will describe a budding effort to change this . After an overview of several known memory behaviors, I will describe some qualitative connections and tentative classifications that attempt to bring together disparate systems. I will pay particular attention to an unusual kind of memory where learning and forgetting are intertwined, observed in sheared non-Brownian suspensions, traveling charge-density waves, and a model of a worn path. Such a comprehensive study of memory has not been pursued before. If it is successful in discerning universal principles, they could apply across scales, from microphysical to astrophysical.
 “Memory formation in matter.” NC Keim, JD Paulsen, Z Zeravcic, S Sastry, and SR Nagel. Rev. Mod. Phys. 91, 035002 (2019).