Here's a breakdown of why:

* Liquids: They have a definite volume but no definite shape. This means they can flow and take the shape of their container.

* Solids: Although they have a definite shape, they can still deform under stress. Imagine bending a metal rod or pressing a rubber ball - it changes shape, albeit slowly. This ability to deform continuously makes them fluids.

Key differences:

While both liquids and solids are fluids, they differ in their response to shear stress:

* Liquids: They flow readily and change shape quickly in response to shear stress. Their molecules can move past each other easily.

* Solids: They resist deformation and change shape slowly in response to shear stress. Their molecules are more tightly packed and resist movement.

In summary:

The term "fluid" encompasses both liquids and solids because they both exhibit the characteristic of continuous deformation under stress, although their responses differ significantly. This concept is important in many fields, including physics, engineering, and materials science.