Here's why:

* Mechanical waves require a medium to travel. This means they need particles (atoms or molecules) to vibrate and transfer energy.

* Solids, liquids, and gases all have particles that are close enough together to allow for this vibration.

* The speed of a mechanical wave depends on the properties of the medium. For example, sound travels faster in solids than in liquids, and faster in liquids than in gases.

Examples of mechanical waves:

* Sound waves are longitudinal waves that travel through solids, liquids, and gases.

* Seismic waves are waves that travel through the Earth's interior. They can be longitudinal (P-waves) or transverse (S-waves).

Electromagnetic waves do not require a medium and can travel through a vacuum (like space). Examples include:

* Light waves

* Radio waves

* X-rays

* Gamma rays