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

The correct answer is Particles are closely packed and vibrate in fixed positions..

Unit lens: Check the units before giving the final statement so the physics quantity is not swapped. This question asks: Why do solids have fixed shapes according to the particle model. The correct response is Particles are closely packed and vibrate in fixed positions., because gas pressure comes from particle collisions with container walls. In Changes of state, the marking point should connect directly to use the particle model to explain why solids have fixed shapes and liquids and gases flow. 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 193 is distinct because it uses this exact question context and the unit lens rather than a generic particle-model sentence.