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 noncrystalline hydrophilic polymers. Upon training with heat in the air and reswelling in aqueous media, super stable and tough hydrogels are readily afforded with the crystalline domains serving as load-bearing phases and physical crosslinks, which endows the gels with satisfactory processability. Various factors, such as training temperature and duration, can effectively regulate the training effect and endows the hydrogels with highly programmable mechanical performance. Upon experiencing another training process with heat in the water, the crystalline domains can be readily disrupted, which endows the materials with good recyclability. Combining these advantages, this strategy is well adaptable for various processing technologies for manufacturing hydrogels, and enables the gels with promising applications such as antifouling hydrogel coatings.