density

Density describes how heavy an object is for its size — formally, the mass packed into a unit volume of a substance.

• A wooden stick and an iron rod of exactly the same size feel very different in the hand because iron packs far more mass into the same volume — that difference is density.
• Density is calculated as Density = Mass ÷ Volume, and it doesn't depend on an object's shape or size, only on what it's made of (though temperature and pressure can shift it).
• Density is a major reason things float or sink: an object less dense than the liquid around it tends to float, while a denser one tends to sink — though density isn't the only factor at play.
• Even Earth itself is layered by density — the crust is the lightest layer, and each layer gets denser moving toward the planet's core, where pressure and temperature are far higher.