Here's why:

* Transverse Waves: In transverse waves, the particles of the medium move perpendicular to the direction the wave travels. Imagine a rope tied to a wall. If you shake the rope up and down, you'll create a wave that moves horizontally along the rope. Each individual point on the rope moves up and down, but the wave itself travels horizontally.

* Circular Motion: Some transverse waves, like those on the surface of water, involve particles moving in a circular path. As a wave passes through the water, the water molecules move in a circular path, both up and down and slightly back and forth.

Important Note: Not all transverse waves involve circular motion. For example, light waves are transverse waves, but the electric and magnetic fields that make up light waves oscillate perpendicular to the direction of wave travel, not in a circular path.