Many organisms encapsulate their embryos in hard, protective shells. While birds and reptiles largely rely on mineralized shells, plants often develop highly robust lignocellulosic shells. Despite the abundance of hard plant shells, particularly nutshells, it remains unclear which fundamental properties drive their mechanical stability. This multiscale analysis of six prominent (nut)shells (pine, pistachio, walnut, pecan, hazelnut, and macadamia) reveals geometric and structural strengthening mechanisms on the cellular and macroscopic length scales. The strongest tissues, found in walnut and pistachio, exploit the topological interlocking of 3D‐puzzle cells and thereby outperform the fiber‐reinforced structure of macadamia under tensile and compressive loading. On the macroscopic scale, strengthening occurs via an increased shell thickness, spherical shape, small size, and a lack of extended sutures. These functional interrelations suggest that simple geometric modifications are a powerful and resource‐efficient strategy for plants to enhance the fracture resistance of entire shells and their tissues. Understanding the interplay between structure, geometry, and mechanics in hard plant shells provides new perspectives on the evolutionary diversification of hard seed coats, as well as insights for nutshell‐based material applications.

In the originally published article, the units for the values of axial stiffness, K, that are presented are incorrect in Figure 4 and Table S1, Supporting Information. The correct units should be kN mm −1 . The units are hereby corrected as shown in the revised Figure 4 and Table S1 below.

In article number 2004519, Jessica C. Huss, Notburga Gierlinger, and co‐workers reveal previously unknown functional interrelations between the structure, geometry, and mechanics in (nut)shells. Their multi‐scale analysis suggests that simple geometric modifications, such as the development of puzzle‐shaped instead of round cells, and small spherical shells instead of large ellipsoidal shells, increase the fracture resistance of hard plant shells resource‐efficiently.