Here's why:

* Solids have the highest density: The molecules in a solid are packed tightly together. This means that the vibrations (which make up the wave) have to travel through a denser medium, leading to slower propagation.

* Strong intermolecular forces: The strong bonds between molecules in a solid resist movement, slowing down the transfer of energy.

Think of it this way: Imagine a line of people holding hands. If they are close together (like in a solid), it takes longer for a push from one end to reach the other.

Conversely, mechanical waves travel fastest in gases because:

* Low density: Molecules are far apart, allowing the wave to travel more easily.

* Weak intermolecular forces: The weak bonds between gas molecules allow for easier vibration transfer.

Let me know if you have any other questions!