What is the main reason solids maintain their shape?

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

Cause lens: Name the cause, then state the effect on particles, pressure, density, or energy. This question asks: What is the main reason solids maintain their shape. The correct response is Particles are closely packed and held 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 196 is distinct because it uses this exact question context and the cause lens rather than a generic particle-model sentence.