Here's why:

* Close Proximity: In solids, atoms are tightly packed together, with very little space between them. This tight arrangement results in strong attractive forces between the atoms.

* Fixed Positions: The atoms in a solid vibrate around fixed positions, unable to move freely like in liquids or gases.

* Definite Shape and Volume: This close packing and fixed positions give solids their definite shape and volume.

Let me know if you'd like more detail about the different types of solids!