* They can flow: This is the most fundamental characteristic. Unlike solids, fluids can move freely and change shape under the influence of an applied force. This is because the molecules in fluids are not fixed in a rigid structure like solids.
* They can't sustain a shear stress: Shear stress is the force applied parallel to a surface. Solids can resist shear stress, maintaining their shape. Fluids, however, deform continuously under shear stress, leading to flow.
* They take the shape of their container: Both liquids and gases fill the container they are in. Liquids have a definite volume, while gases expand to fill the entire container.
In summary: The ability to flow and the inability to sustain shear stress are the defining characteristics of fluids. These properties make them distinct from solids, which can hold their shape and resist shear stress.
