Hello
The level portions in the graph indicates phase change .
When water is heated from a sub-zero degree celcius to zero degree celcius, the phase changes from solid to liquid. This change is usually as a result of excitment of the molecules due to heat and the bond between them slackens or become weaker. The second phase change is noticed at 100 degree celcius. This is as a result of when water changes state from liquid to vapour or gaseous state. This is primarily as a result of heat on the molecules and they begin to move randomly and colliding with the walls of the container.
From the explanation above, the answer to this question is phase change