Here's why:

* Energy transfer: The oscillating motion of a periodic wave causes particles in the medium (or in the case of electromagnetic waves, the oscillating electric and magnetic fields) to vibrate. This vibration transfers energy from one point to another without the actual movement of matter over large distances.

* No matter transfer: While the particles in the medium move back and forth, they don't travel along with the wave. Think of a ripple in a pond – the water molecules move up and down, but they don't travel across the pond with the wave.

Examples:

* Water waves: The water molecules move up and down, but the wave itself travels across the surface of the water.

* Sound waves: Sound waves travel through air by causing air molecules to vibrate back and forth.

* Light waves: Light waves are electromagnetic waves, and they transfer energy through oscillating electric and magnetic fields.

Let me know if you have any other questions!