These are called Phase Transitions.
Solids become liquids when the temperature reaches melting point, groups of particles start to brake away from each other. This is when the solid starts becoming a liquid. Liquids become solids on...
solid-solid
Butter is a solid. By definition of a solid, butter has a definite shape and a definite volume.
Peanut butter is a solid. By definition of a solid, peanut butter has a definite volume and definite shape.