Here's why:

* Solids: The molecules in solids are tightly packed together, allowing vibrations to transfer quickly and efficiently from one molecule to the next.

* Liquids: Molecules in liquids are more spread out than in solids, so vibrations take slightly longer to transmit.

* Gases: Molecules in gases are very far apart, resulting in the slowest transmission of sound waves.

In summary: The speed of sound is directly related to how tightly packed the molecules are in a substance. The denser the medium, the faster sound travels.