Pressure of fitness ball

When you squeeze the ball hard with your hands, the volume of the ball will become smaller, that is, the air inside will be squeezed. At this time, the volume will decrease and the mass will remain unchanged, but the density will remain unchanged, so the mass of air will change with the volume at this time, so the volume will decrease and the natural mass will decrease. If there is no air in it, the ball will naturally contract.