Congratulations to Shu and co-authors! Their paper, “Facile mechanochemical cycloreversion of polymer cross-linkers enhances tear resistance” has been published in Science 2023, 380, 1248-1252. Shu’s work holds fundamental implications for our understanding of the mechanisms of toughening in polymer networks, and there are practical implications as well. Polymer engineers know how to make materials tougher, but it invariably involves changing some other property of the material that you often don’t want to change (like its Young’s modulus, and especially its viscous loss modulus). In the mechanism at play here, the toughness enhancement comes without any other significant change in physical properties (at least that we can measure), and it is brought about through the replacement of only a small (few%) fraction of the overall material.