The production of natural materials is achieved through a bottom-up approach, in which materials spontaneously grow and adapt to the external environment. Synthetic materials are specifically designed and fabricated as engineered materials; however, they are far away from these natural self-growing attributes. Thus, design and fabrication of synthetic material systems to replicate the self-growing characteristics of those natural prototypes (i.e., hairs and nails) remains challenging. Inspired by the self-growing behaviors of keratin proteins, here we report the fabrication of synthetic hydrogels (i.e., polyacrylamide, PAAm) from the free radical polymerization at the interface between AAm precursor solution and liquid metals (i.e., eutectic gallium-indium, EGaIn). The newly-formed hydrogel materials at the EGaIn/AAm precursor interface gradually push the whole hydrogel upwards, enabling the self-growing of these synthetic hydrogel materials. This work not only endows the fabrication of synthetic materials with unprecedented self-growing characters, but also broadens the potential applications of self-growing materials in actuation and soft robotics. This article is protected by copyright. All rights reserved.