Freida wants to model the way atoms move when a substance changes its state. To do this, Freida makes a pyramid of marshmallows. Then, she knocked down the pyramid causing the marshmallows to fall.

The marshmallows start in an orderly, tightly packed arrangement, like particles in a solid. When the pyramid is knocked down, they move out of that fixed arrangement, which models melting: particles in a solid gain freedom to move as it changes into a liquid. The correct choice is melting.

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