This paper demonstrates that it is straightforward to develop the theory of an atom in a moleculethe extension of quantum mechanics to an open systemby deriving the necessary equations of motion from Schrodinger's equation, followed by a comparison of the predicted properties with experiment to determine the correct boundary condition. Although less fundamental than the variational derivation of the quantum theory of atoms in molecules, this heuristic approach makes the quantum mechanics of an atom in a molecule accessible to "everyman" possessing a knowledge of Schrodinger's equation, aiding its general acceptance by experimental chemists.