Here's why:

* Energy is used to break or form bonds: Phase changes involve breaking or forming intermolecular bonds. For example, when ice melts, the energy supplied goes into breaking the hydrogen bonds holding the water molecules in a solid structure.

* No increase in kinetic energy: This energy input does not increase the kinetic energy of the molecules (which is what we perceive as temperature). Instead, it's used to change the state of matter.

* Plateau on heating curves: This constant temperature during phase change is reflected in the plateau seen on heating curves, where the temperature remains stable even though heat is still being added.

Examples:

* Melting: Ice melting into water.

* Boiling: Water boiling into steam.

* Freezing: Water freezing into ice.

* Condensation: Steam condensing into water.

Let me know if you'd like more detail on any of these processes!