* Intermolecular forces: Solids are held together by strong intermolecular forces, which are the attractive forces between molecules. These forces can be:

* Van der Waals forces: These are weak attractions that arise from temporary fluctuations in electron distribution around molecules. They are present in all solids but are stronger in larger molecules.

* Hydrogen bonding: This is a special type of dipole-dipole interaction where a hydrogen atom is bonded to a highly electronegative atom (like oxygen or nitrogen). It's a particularly strong intermolecular force.

* Dipole-dipole forces: These arise from the permanent dipoles in polar molecules.

* Ionic bonds: In ionic solids, the attraction between oppositely charged ions is very strong. This is the electrostatic force holding the ions together.

* Metallic bonds: In metals, the electrons are delocalized and form a "sea" of electrons that hold the metal ions together. This strong attraction gives metals their characteristic properties like conductivity and malleability.

Key takeaway: The strong attractions between particles in a solid are what give them their fixed shape and volume. They resist changes in shape and volume much more than liquids or gases.