Here's why:

* Longitudinal waves: The particles of the medium vibrate parallel to the direction the wave travels. Think of a slinky – when you push one end, the coils compress and expand, creating a wave that moves along the slinky. The compression and expansion are parallel to the direction the wave moves.

* Transverse waves: The particles of the medium vibrate perpendicular to the direction the wave travels. Imagine a rope tied to a post. If you shake the rope up and down, you create a wave that travels along the rope, but the rope itself moves up and down (perpendicular to the wave direction).

Examples of longitudinal waves:

* Sound waves: Sound travels through air as compressions and rarefactions (areas of high and low pressure) which are parallel to the direction of sound propagation.

* Seismic P-waves: These waves travel through the Earth's interior and cause the ground to vibrate back and forth.

Let me know if you have any other questions!