Sound can propagate through a medium such as air, water, and solids as longitudinal waves and also as a transverse wave in solids. Sound travels about four times faster and farther in water than it does in air. The distances between molecules in solids are very small, i.e., solids are denser - as compared to liquids and gases. Because they are so close, they can collide very quickly, i.e. it takes less time for a molecule of the solid to 'bump' into its neighbour. Solids are packed together tighter than liquids and gases, hence sound travels fastest in solids. The distances in liquids are shorter than in gases but longer than in solids. Liquids are denser than gases but less dense than solids, so sound travels 2nd fast in liquids. Gases are the slowest because they are the least dense: the molecules in gases are very far apart, compared with solids and liquids.