Heavier Molecule

Which is heavier, a molecule of water or a molecule of air?

Hint: A molecule of water is composed of one atom of oxygen and two atoms of hydrogen.

Answer:

A molecule of water, because there is no such thing as a molecule of air! The air is simply a mixture of various gases. The air contains molecules of oxygen, molecules of nitrogen, molecules of carbon dioxide and molecules of various other gases -- but there is no such thing as a molecule of air.