Metallic materials used in engineering must be strong and ductile—capable of carrying high mechanical loads while able to withstand deformation without breaking. Whether a material is weak or strong, ductile or brittle, however, is not determined simply by the crystal grains that make up the material, but rather by what happens in the space between them known as the grain boundary. Despite decades of investigation, atomic-level deformation processes at the grain boundary remain elusive, along with the secret to making new and better materials.