The physical property of a substance tends to be defined by its melting point as it relates to nature. However, if its normal state is as a solid, the environment needs to be changed to make it liquid. For instance, water at what humans consider normal temperature exists as a liquid. Yet when colder, it solidifies into ice. As a result, to melt something that in its normal state exists as a solid, the surrounding environment of that object needs to be changed (typically by raising the temperature).
The most common obstacle to achieving a melting point involves raising the surrounding temperature to a hot enough level to change the molecular makeup of the material. At the atomic level the molecules need to be sped up enough to lose their normal structured integrity that make the substance a solid. If the ambient temperature won't suffice, then some kind of applied heat needs to be used. It also needs to be hot enough to generate the appropriate temperature even when the solid may be cooler. Frequently flame is used for this purpose.
For a solid to properly melt, the change in environment needs to be contained. Otherwise, the energy involved escapes to the surrounding space and air. This is why foundries, for example, melt iron ore inside a container. The heat stays in and the melted iron doesn't flow away once it becomes liquid. Since the conditions are extreme, the container needs to be made of material strong enough to handle the conditions and remain solid itself.
Some materials will have a higher resistance to melting than others. Clearly steel and wood take more heat to burn than an ice cube. Overcoming this resistance means raising the applied heat or energy involved to the point where the solid actually changes to a liquid. The more dense the material, such as steel, the hotter it will need to be to melt.
Irregular melting effects occur when a solid is heated unevenly. This frequently occurs when a single point of heat is applied versus a surround heat application (blow torch versus oven). When the heat application varies, some of the solid could melt quickly while a cooler portion remains solid. Particularly with metals, this sort of consequence can become a problem in production processes.