What is Melting point?

Example 21.3 (Reading the world with two numbers)

Why is a chocolate bar solid on the shelf ($20\,{}^{\circ}\mathrm{C}$) but liquid in your pocket on a summer hike ($35\,{}^{\circ}\mathrm{C}$)? Because its melting point, near $34\,{}^{\circ}\mathrm{C}$, sits between the two. A stuff is solid below its melting point, liquid above it — up until its boiling point, where it leaves as gas. Two numbers tell the whole story of any stuff at any temperature.

Example 21.9 (The candle and the foundry)

A candle by the fireplace slumps and drips: wax passes its melting point near $60\,{}^{\circ}\mathrm{C}$. In a foundry, workers pour glowing-orange liquid iron like soup — their furnace passed $1538\,{}^{\circ}\mathrm{C}$. Same game, wildly different milestones: what the fireplace does to wax, only a furnace can do to iron. And gold melts at $1064\,{}^{\circ}\mathrm{C}$: goldsmiths have known that number, in their hands if not in degrees, for thousands of years.