An analogy makes a comparison between objects based on their similar qualities. cassidy wanted to create an analogy for the motion of atoms in solids, liquids, and gases, so she compared them to marbles in a tray.

Motion of solids, liquids, and gases may be compared with the motion of marbles in a tray: molecules of solids do not move one relative to each other, like the marbles in a tray full, molecules of liquids are like marbles in a tray with many marbles but no full, and molecules in gases are like a few marbles in a tray.

As you can imagine, as the tray has less empty space (the molecules are closer and tighter) the marbles cannot move freely, which is like the molecules of a solids which cannot move one relative to each other; when the tray has some room to move, the marbles can move better, which is like the molecules of a liquid, and when there is plenty space for marbles to move in the tray is like the molecules of gases which move freely without interacting among them.