Here's why:

* Sound travels as vibrations: Sound waves are vibrations that travel through a medium by causing molecules to oscillate.

* Density affects vibration speed: Denser materials have molecules packed closer together. This means the molecules can collide and transfer energy more quickly, leading to a faster propagation of the sound wave.

Here's a simplified explanation:

Imagine a line of tightly packed bowling pins. If you knock over one, the energy transfers quickly to the next pin, and so on, making the vibration travel quickly. Now imagine the pins are spaced far apart. The energy transfer will be slower as it takes longer for the pins to collide.

In summary:

* Higher density = faster sound velocity

* Lower density = slower sound velocity

Note: While density is a major factor, the velocity of sound also depends on other factors like the elasticity (ability to return to its original shape) of the medium and temperature.