Most things around us are made of groups of atoms bonded together into packages called molecules. The atoms in a molecule are held together because they share or exchange electrons.
Molecules are made from atoms of one or more elements. Some molecules are made of only one type of atom. For instance, two oxygen atoms bond together to form O2, the part of the air that we need to breath to survive. Other molecules are very large. Protein molecules, for example, often contain hundreds of atoms.
Even large molecules are so small that you would not be able to see one molecule of a substance. But when thousands of molecules are together, they might look like a glass of water, a tree in the forest, or your computer screen, depending on what types of molecules they are.
Even if a soccer ball is sitting still, the molecules within it are moving all the time. They may be too small to see, but molecules are always moving and
source: windows to the universe