Why do liquids take the shape of their containers while keeping a definite volume?

Liquids maintain a definite volume while conforming to the shape of their container because of their unique molecular structure and behavior. Unlike gases, which expand to fill any available space, liquids are a form of condensed matter where molecules remain in close proximity to one another, resulting in high density.12

On a molecular level, this behavior is defined by the following characteristics:

• Molecular Mobility: Unlike solids, where atoms are fixed in a durable, lattice-like structure and only oscillate about a point, molecules in a liquid are mobile and continually wander throughout the material. This lack of an enduring spatial arrangement allows a liquid to flow under external forces.2
• Resistance to Shear Stress: A key distinction between states of matter is that a solid exhibits resistance to shear stress, allowing it to hold its shape, whereas a liquid does not. Because liquids cannot withstand shear stress, they flow until they conform to the shape of their vessel.2
• Intermolecular Forces: While molecules in a liquid are free to move, they remain relatively close to one another due to attractive forces. This "partial order" ensures that the substance retains a constant volume, as the molecules are packed almost against one another, preventing the expansion seen in gases.31