Why do solids have fixed shapes according to the particle model?

The correct answer is Particles are tightly packed and can only vibrate in place.

Exam lens: Write the relationship, substitute values only when needed, and finish by interpreting the result. This question asks: Why do solids have fixed shapes according to the particle model. The correct response is Particles are tightly packed and can only vibrate in place, because gas pressure comes from particle collisions with container walls. In Changes of state, the marking point should connect directly to explain sublimation as a change directly between solid and gas. If the question includes values, the working must keep the appropriate unit and operation; if it is an explanation, it must name the relevant particle behaviour or energy change. This item belongs to Changes of state and the particle model, so avoid answers that switch to a different quantity, confuse heat with temperature, or describe gas pressure without collisions when collisions are the reason. Checkpoint 155 is distinct because it uses this exact question context and the exam lens rather than a generic particle-model sentence.