variations, thereby becoming significantly toughened and enabling them for various application scenarios. [1] Among the various materials under training, water-based hydrogels have attracted intense attention in numerous engineering [2,3] and biological applications. [4,5] As 3D crosslinked networks infiltrated with a large amount of water, hydrogels demonstrate various similarities to biological tissues, such as hydrophilicity, biocompatibility, and flexibility. [6,7] To meet the demand for practical applications, hydrogels with satisfactory toughness have attracted increasing attention in the past few decades. [8][9][10][11] Similar to other artificial materials, the training of hydrogels can enable better mechanical properties. For example, inspired by human tissues that autonomously grow and remodel themselves to adapt to the surrounding mechanical environment, Gong et al. have convinced innovative self-growing polymeric hydrogels that can be reinforced to repetitive mechanical stress through effective mechanochemical transduction. [12,13] Similarly, novel muscle-like hydrogels after mechanical training can give rise to aligned nanofibrillar architectures to resist fatigue. [14] Likewise, a soft and single network hydrogel can afford super high mechanical performance by forming double network composite structures after experiencing light treatment. [15] Hydrogels can also be trained at low temperatures to render better rigidity and extensibility, such as semicrystalline polyvinyl alcohol (PVOH) and polysaccharide gels. [16,17] Despite numerous innovations in hydrogel training, they are still far from satisfactory for industrial applications. First of all, the majority of current training approaches lack reversibility, which typically yields unrecoverable strengthening performance. Taking the aforementioned self-growing hydrogels and light-trained hydrogels, for example, once the gels are reinforced after training, they cannot easily recover to their initial states. This one-way performance limits its recyclability and imposes environmental stress. Second, the hash and timeconsuming training processes often lead to complications and difficulties in manufacturing. The semicrystalline hydrogels, for instance, have to undergo several days of freezing-thawing cycles at the subzero degree to fulfill the training process. [17] This tedious procedure is merely acceptable for scientific research but hardly meets the requirement for practical constructions. In certain cases, the training process even requires Manufacturing hydrogels with programmable and reversible mechanical performance is pivotal for practical applications. In this work, a simple yet effective strategy is developed to fulfill this goal by training hydrogels with heat in different environments. The key point is to delicately tune the formation of the crystalline domains, thus reversibly switching the materials from liquid to solid, or from soft to rigid. For illustration, hydrogel precursors are taken with soluble semicrystalline polyvinyl alcohol and...
