In the simple particle model, particles are drawn as small, solid, inelastic spheres, a bit like tiny bowling balls. The model is useful because it explains states of matter and changes of state, but it is only a model and has limitations.
First, the model does not take into account the forces of attraction between particles. Bowling balls do not attract each other, yet real particles do, and the strength of these forces is what decides how much energy is needed to melt or boil a substance. The model cannot explain why different substances have different melting and boiling points.
Second, the model ignores the size of particles. Real particles are atoms or groups of atoms, and they are not all the same size or shape. Third, it ignores the space between particles. The spheres are drawn touching in solids and liquids and far apart in gases, but the model does not show how big that gap really is, or that it is empty space.