In the phase diagram the line between solid and liquid is where both phases available at the same time
I believe the answer is a balance or mechanical scale
Answer:
to attain stability atoms form covalent bonds
1. slowly heat stubstance.
2. once the substance is at the most liqued state take the temp. that's the melting point of that subtance.
hope that helps, any other questions feel free to DM me dont wate your points. :)