Final answer:
The shape of a liquid can change because its particles have enough energy to move past each other but still remain in close contact, allowing liquids to assume the shape of their containers while maintaining a definite volume.
Step-by-step explanation:
The best explanation for why the shape of a liquid can change is that the particles have just enough energy to move past each other. In the liquid state, particles are still in close contact, thus liquids have a definite volume. However, they can move about each other rather freely, which allows a liquid to have no definite shape and take on the shape of its container. The particles are not bound in fixed positions, unlike a solid, and are not completely free from each other's influence, unlike a gas, which makes them unique in their ability to flow and change shape.