Here's why:

* Longitudinal waves: The particles in the medium vibrate parallel to the direction the wave travels. Think of a spring: when you push one end, the coils move back and forth in the same direction the wave travels.

* Transverse waves: The particles vibrate perpendicular to the direction the wave travels. Think of a rope: when you shake it up and down, the waves travel horizontally, but the rope itself moves up and down.

Examples of longitudinal waves include:

* Sound waves: Sound travels through air, water, or solids by causing the particles to vibrate back and forth.

* Seismic waves (P-waves): These are the first waves to arrive from an earthquake and travel through the Earth's interior by compressing and expanding the rock.