The particles that move or slide past each other are the atoms or molecules in a fluid. Fluids include:

* Liquids: In liquids, the particles are close together but can move around each other. This is why liquids can flow and take the shape of their container.

* Gases: In gases, the particles are much farther apart and move around freely. This is why gases can expand to fill any space.

Solids, on the other hand, have particles that are tightly packed together in a fixed arrangement. They vibrate in place, but they don't slide past each other.

So, the movement or sliding of particles is a key characteristic of liquids and gases.