Here's why:

* Particle Spacing: Gas particles are much farther apart than solid particles. This means there is more space between gas molecules for them to move around.

* Kinetic Energy: When heated, both gases and solids gain kinetic energy (energy of motion). But because gas molecules are already moving freely and have more space to move, they can expand more significantly.

Think of it this way:

* Imagine a crowded room full of people (solid). If you add more people (heat), they will push against each other, but there's limited space to move around.

* Now imagine a large, empty field (gas). If you add more people (heat), they have much more room to move around and spread out.

Key Points:

* Thermal Expansion: Both gases and solids expand when heated, but gases expand much more significantly due to their greater particle spacing.

* Density: Solids are much denser than gases because their particles are packed more tightly together. This means there's less room for expansion.

Let me know if you have any other questions!