Sink or float?

Learning objectives

To classify objects according to whether they sink or float.
To introduce the notion of density (middle school).
To teach that a floating body displaces a volume of fluid equal in mass to the mass of the body.

Archimede's principle: an object immersed in fluid is buoyed up by a force equal to the weight of the fluid displaced by the object.
You can choose between water or alcohol for the testing liquid. Materials that are less dense than the testing liquid will float.
The density of (fresh) water is 1.0 grams per cubic centimeter (g/cm3) and alcohol is about 0.8 g/cm³.
If you consider the following densities:

ice is 0.9 g/cm³
walnut is 0.5 g/cm³
stone is 2 g/cm³

then you will be able to explain why the ice cube floats in water and sink in alcohol.

Safety precaution: alcohol is a flammable liquid; keep away from all flames.