If a substance is pure, then it will either be a solid, a liquid or a gas, depending on how hot it is. So, for example, ice suddenly changes to water at 0 degrees C and water changes to gas (steam) at 100 degrees C.

But most materials are not like that because they are not pure substances like water, but mixtures of a number of substances. Chocolate, butter and the spread you might use on your bread are like that. They consist of a number of substances that have been blended (mixed) together, and each one melts at its own melting point. This means that part of the mixture will melt before other parts.

As soon as one part melts, it stops holding tight to the other parts, and so the mixture is easier to move about even though it still looks like a solid. This is called softening. Complete melting only occurs when each substance in the mixture has melted.