Take a stick of butter and cut off about a tablespoon of the stick. Take the rest of it and put it in a microwaveable dish and melt it.
Now put the pat of butter that you had taken off and plop it in the melted butter.
It should sink to the bottom of the melted butter.
Now take an ice cube and plop it into a cup of water.
The ice floats, right?
Why do they act differently?
With nearly every substance in the world the molecules get closer together when the substance turns solid, and get farther apart when it turns into a liquid. That is how the solid butter pushed its way through the liquid butter. However, with the water, the hydrogen atoms on one molecule try to make up for their lack of electrons by sharing some of the extra electron charge found on the oxygen atom of the another water molecule. Since the oxygen has only a small excess of electron this is a weak bond. It is called a hydrogen bond. When water is a solid its molecules cannot hydrogen bond. When it becomes a liquid its molecules are free to move close enough together to hydrogen bond. As a result water has the unique characteristic that its molecules are closer together when in a liquid state than when in a solid state. Because of this the ice cube could not push its way through the water molecules and instead floated on top.
source: Exploring Creation with Physical Science, Jay Wile