Molecules are often represented by simple ball and stick models in which the balls stand for atoms and the sticks stand for the chemical bonds that bind the atoms together to form the molecules. This is only an analogy; atoms aren't exactly like balls and chemical bonds certainly aren't exactly like sticks. The following image illustrates a schematic model of a water molecule.

Molecules have additional degrees of freedom relative to atoms that can absorb energy. In particular, a molecule can rotate, and it can vibrate either the length of its chemical bonds, or their orientation angles. The arrows on the preceding image illustrate the possible vibrational modes of excitation for the water molecule.