It depends on the definition of inertia: we call the property that an object keeps its state of motion unchanged inertia. Inertia indicates the difficulty of changing the motion state of an object. The size of inertia is only related to the mass of the object. The motion state of a mass object is difficult to change, that is, the inertia is large; The motion state of objects with small mass is easy to change, that is, the inertia is small. The inertia of liquid and solid is only related to its mass. For example, when liquids and solids of the same mass are at rest, they begin to move under the action of force F. If the external friction is ignored, when F is vacuumized, the liquid will fluctuate, resulting in the change of the overall center of mass position, and the speed of its container to the ground is non-uniform. However, if the kinetic energy consumed by the friction between the liquid molecule and the container is ignored, theoretically, its centroid velocity is uniform and the velocity is the same as that of a solid with the same mass. That is to say, the difficulty of changing its motion state is the same. Which is inertia.