Final answer:
When molecules in a material gain more energy, they can change from one phase to another.
Step-by-step explanation:
The molecules in a material need to move with more energy for the material to change from one phase to another. When energy is added to a material, such as by heating it, the molecules gain kinetic energy and begin to move faster. This increased movement can overcome the attractive forces between the molecules and cause them to change their arrangement, leading to a phase change. For example, heating a solid material like ice increases the energy of its molecules, causing it to melt and become a liquid.