This paper presents a novel method to measure the superfluid fraction of a supersolid using the Josephson effect. Supersolids are a unique phase of matter characterized by spontaneous breaking of gauge and translational symmetries, leading to spatially modulated macroscopic wavefunctions. The superfluid fraction, which quantifies the reduction in superfluid stiffness due to spatial modulation, has been introduced but not experimentally assessed. The study demonstrates that individual cells of a supersolid can sustain Josephson oscillations, and from the current-phase dynamics, the superfluid fraction can be derived directly. Using a cold-atom dipolar supersolid, the researchers find a sub-unity superfluid fraction, indicating the presence of partially quantized vortices and supercurrents. This work opens new research directions, potentially unifying the description of all supersolid-like systems and providing insights into the fundamental properties of supersolids.This paper presents a novel method to measure the superfluid fraction of a supersolid using the Josephson effect. Supersolids are a unique phase of matter characterized by spontaneous breaking of gauge and translational symmetries, leading to spatially modulated macroscopic wavefunctions. The superfluid fraction, which quantifies the reduction in superfluid stiffness due to spatial modulation, has been introduced but not experimentally assessed. The study demonstrates that individual cells of a supersolid can sustain Josephson oscillations, and from the current-phase dynamics, the superfluid fraction can be derived directly. Using a cold-atom dipolar supersolid, the researchers find a sub-unity superfluid fraction, indicating the presence of partially quantized vortices and supercurrents. This work opens new research directions, potentially unifying the description of all supersolid-like systems and providing insights into the fundamental properties of supersolids.