Sound waves, like all other waves, are transmitted through amedium by having the particles in the medium actually collide witheach other, transmitting the vibration even further. The speed ofthe wave is proportional to the number of collisions, so the morecollisions that can be made the faster the wave can travel. Solidsby definition have the closest packed particles, so there can bemore collisions in this phase, meaning that solids also have thefastest sound waves. Likewise, gasses have the most spread outparticles, so their infrequent collisions mean that gasses have theslowest sound waves. Liquids are halfway between solids and gassesin terms of particle packing, so their speed is in between.For example, sound travels at around 343 m/s in the air, 1,484m/s in water, and 5,120 m/s in solid iron!