The inter-electron-pair Coulomb repulsion and the segmental-thermodynamic-disparity of the master-slave-segmented O^2-:H^+/p–O^2- (H) bond are shown to originate the density anomalies of water ice. Consistency between Raman measurements, MD computations, and modeliing expectations revealed the following: In the liquid (I) and solid (III) phases, the softer non-bond “:” serves as the “master” (of lower specific heat, see below) that contracts largely and meanwhile forces the stiffer real-bond “-” as “slave” into Coulomb-repulsion-driven slight elongation, leading to the O^2-:H^+/p–O^2- cooling contraction and the seemingly normal cooling densification; at transition phase (II), the master-slave swap roles, resulting in the O^2-:H^+/p–O^2- freezing elongation and volume expansion. The O^2-:H^+/p–O^2- of ice is longer than that of water, and therefore, ice floats. In addition, angle relaxation also contributes to the volume change during the process of relaxation.