This document discusses volume and density in O-Level Physics. It defines volume and describes the displacement method for measuring the volume of irregular objects that are heavier or insoluble in water. The displacement method involves fully immersing the object in water and measuring the rise in water level, which equals the object's volume. It also defines mass and density, noting density is the ratio of an object's mass to its volume. The formula for density is provided along with examples of water and gold's densities.