Here's why:

* Solids: Particles in solids are tightly packed together and have a fixed, regular arrangement. This close proximity and order lead to strong intermolecular forces (forces of attraction between molecules) that hold the particles firmly in place. These strong forces restrict movement, giving solids their rigid shape and incompressibility.

* Liquids: Particles in liquids are less tightly packed than in solids and have a less ordered arrangement. The intermolecular forces are weaker in liquids, allowing particles to move more freely and take the shape of their container. While the forces are weaker than in solids, they are still strong enough to hold the particles together, preventing the liquid from expanding infinitely.

In summary: The strength of intermolecular forces determines the state of matter. Solids have strong intermolecular forces, liquids have weaker intermolecular forces, and gases have very weak intermolecular forces.