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.
