Do you know the molecular structure of the water molecule? I believe that you know it well. However, if I ask you to compare the molecular structure of light water (H2O) with that of heavy water (D2O), you might be at a loss. Can we say that both light water and heavy water have the same molecular structure? In the same way, can we say that both H2 and D2 molecule have the same molecular structure? The Schrödinger equation in quantum mechanics predicts that both H2 and D2 have the same structure. From the NIST Chemistry WebBook, however, we have that the H-H bond length is 0.74144 Å and the D-D bond length is 0.74152 Å. Yes, there might be a fallacy in quantum mechanics. In this book, it will be discussed to unveil whether the heavy water has the same molecular structure as the light water. In order to know quantum mechanics better, it is necessary to do "seeing the forest for the trees". Hence I introduce firstly how quantum mechanics is related to classical mechanics, thermodynamics, and statistical thermodynamics. In the end, I provided the nuclear quantum effect on the molecular structure of water and the molar volume of ice.