Here's why:

* Ordered Structure: Crystalline solids have a highly ordered, repeating arrangement of atoms, ions, or molecules in a three-dimensional lattice structure. This structure is responsible for their characteristic properties, including their shape.

* Regular Geometry: The repeating arrangement leads to the formation of specific geometric shapes with well-defined faces, edges, and angles. These shapes are often referred to as crystals.

* Anisotropy: Because of their ordered structure, crystalline solids often exhibit different properties in different directions. This property, known as anisotropy, can influence their shape.

Examples:

* Salt crystals: Cube-shaped

* Quartz crystals: Hexagonal prisms

* Diamond crystals: Octahedral

Exceptions:

While most crystalline solids have a definite shape, there are some exceptions.

* Polycrystalline materials: These materials are composed of many small crystals that are randomly oriented, resulting in an irregular overall shape.

* Amorphous solids: These solids lack the long-range order of crystalline solids, so they don't have a definite shape. Examples include glass and rubber.