Density is the mass per unit volume:
ρ = m/V
Solids are generally denser than liquids, which are denser than gases.
To measure density: regular solid — use ruler for dimensions, calculate volume. Irregular solid — water displacement in a measuring cylinder. Liquid — measure mass and volume directly.
Particle model: solids have particles in a regular arrangement vibrating in place. Liquids have close but irregular particles sliding past each other. Gases have widely spaced particles moving fast and randomly.