(Part of this is from Wikipedia. They explain the ideas simply and the math is correct.)

The mass density or density of a material is its mass per unit volume. Mathematically, density is defined as mass divided by volume:

D=m/V

where D is the density, m is the mass, and V is the volume.

We're leaving out the effects of temperature and pressure for this simple explanation.

Dense fluids float on more dense fluids if they do not mix. This concept can be extended, with some care, to less dense solids floating on more dense fluids. See the graphic below for a demonstration:

liquids of different density in layers in a tube with solid objets of different densities floating inside

posted on reddit Oct 2012 by username niglet_please

If an object is less dense than water it will float in water. If it is denser than water it will sink in water.

Density Simulation from PhET, University of Colorado, Boulder. Interactive, quickly shows the concept of density using objects of different mass like ice cube, brick, etc. Some float, some sink. One of many such simulations from PhET. This simulation used to use Flash. Now it uses HTML5.
Ignore any Flash warnings you see. Just click the link.

Rainbow Jar A Kids' Science Experiment. Written by a National Board-certified Kindergarten teacher. For pre-K and K-1 learners. Teaches the concept of density. Use common household ingredients to make a rainbow jar. Article includes the materials list, recipe, photos, and lesson plan.

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