Fact 1: Melting means changing from a solid into a liquid by adding heat energy.

Fact 2: Melting is a phase change.

Given the two facts above, answer the question "What changes a solid into a liquid using heat energy?" with the following options: 
- electricity
 - It gets heated up
 - boiling
 - aqueous solution
 - a phase change
 - a hydraulic system
 - heating liquids
 - Water expanding


 

     a phase change    