How does a solid differ from a liquid?

The distinction between solids and liquids fundamentally lies in their arrangement and behavior of particles. Solids possess a definite shape and volume because their particles are tightly packed in a fixed formation, allowing them to maintain their shape without the influence of external forces. On the other hand, while liquids also have a definite volume, their particles are less tightly packed and can move more freely. This flexibility allows liquids to adapt to the shape of their container, resulting in the characteristic behavior of flowing to fill the available space.

The other choices do not accurately represent the fundamental differences. For example, weight comparisons between solids and liquids can vary widely depending on the materials being compared, making the idea that solids are always heavier inaccurate. The notion that solids can flow like liquids is also incorrect, as it is liquids that exhibit this property. Lastly, the statement about molecular composition mischaracterizes the arrangement; while both solid and liquid molecules can be close together, it is the fixed arrangement in solids that defines their shape, not the mere proximity of particles.