Why are there two plateaus on the heating curves for water that begins at -15c and ends at 120c?

The two plateaus on the heating curve for water represent phase changes: the first plateau occurs during the melting of ice into water at 0°C, and the second occurs during the boiling of water into steam at 100°C. During these phase changes, the temperature remains constant as the heat energy is used to break the intermolecular bonds rather than increase the temperature. The initial plateau reflects the energy required to overcome the solid-state bonds, while the second plateau corresponds to the transition from liquid to gas.