Density is the mass of an object divided by its volume (ρ = m/V). It's typically measured in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). Water has a density of exactly 1.0 g/cm³ at 4°C (39°F), which serves as the reference point for determining whether everyday objects float or sink.

Objects float when their density is less than the density of the liquid they're placed in. According to Archimedes' Principle, an object immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces. If the object's density is lower than water (1.0 g/cm³), the buoyant force exceeds its weight, and it floats. If its density is greater, it sinks.

Ice has a density of about 0.92 g/cm³, which is lower than liquid water (1.0 g/cm³). This is unusual because most substances are denser in their solid state. Water expands when it freezes due to hydrogen bonding forming a crystalline lattice structure, making ice less dense. This property is crucial for aquatic life—ice floats and insulates the water below, preventing lakes and oceans from freezing solid.

An unpeeled orange (density ~0.75 g/cm³) floats because its thick, porous peel contains numerous tiny air pockets that significantly reduce its overall density. Once peeled (density ~1.05 g/cm³), the denser fruit flesh causes it to sink. This is a classic demonstration of how trapped air can dramatically affect an object's buoyancy!

The human body has an average density of approximately 0.98 g/cm³ with lungs full of air, which is slightly less than water—so most people can float, especially in saltwater. However, factors like body composition (fat is less dense than muscle), lung capacity, and water salinity all affect buoyancy. In the Dead Sea (density ~1.24 g/cm³), nearly everyone floats effortlessly!

Although steel has a density of 7.8 g/cm³, a ship's hull is hollow and contains mostly air. The average density of the entire ship (steel + air + cargo) is less than 1.0 g/cm³, allowing it to float. This is Archimedes' Principle in action—the ship displaces a volume of water weighing more than the ship itself. If water enters the hull, the average density increases and the ship sinks.

A fresh egg has a density of about 1.03 g/cm³—just slightly more than water—so it sinks. However, as an egg ages, moisture evaporates through the porous shell and air enters, decreasing its density. A very old egg may float! This is why the "egg float test" works: if an egg floats in water, it's likely past its prime.

Several common liquids are denser than water (1.0 g/cm³): Honey (~1.4 g/cm³), corn syrup (~1.37 g/cm³), whole milk (~1.03 g/cm³), and cola/soda (~1.04 g/cm³ due to dissolved sugar). Less dense liquids include olive oil (~0.92 g/cm³), rubbing alcohol (~0.79 g/cm³), and gasoline (~0.71 g/cm³). This is why oil floats on water!