It has long been puzzling that water nanodroplet undergoes simultaneously "supercooling" at freezing and "superheating" at melting. Recent progress [Sun et al., J Phys Chem Lett 2013, 4: 2565; ibid, 4: 3238] enables us to resolve this anomaly from the perspective of hydrogen bond (O:H-O) specific-heat disparity. A superposition of the specific-heat x(T) curves for the H-O bond (x = H) and the O:H nonbond (x = L) defines two intersecting temperatures that form boundaries of the quasi-solid phase between ice and liquid water. Molecular undercoordination (with fewer than four nearest neighbours in the bulk) stretches the H(T) curve by raising the Debye temperature DH through H-O bond shortening and phonon stiffening. The H(T) stretching is coupled with the L(T) depressing because of the Coulomb repulsion between electron pairs on oxygen ions. The extent of dispersion varies with the size of a droplet that prefers a core-shell structure configuration - the bulk interior and the skin. Understandings may open an effective way of dealing with the thermodynamic behaviour of water droplets and bubbles from the perspective of O:H-O bond cooperativity.