Here's why:

* Mechanical waves require a medium to travel. They are disturbances that propagate through a material (like air, water, or a solid). Examples include sound waves, waves on a string, and seismic waves.

* Radio waves are electromagnetic waves. They are made up of oscillating electric and magnetic fields that can travel through a vacuum (like space) as well as through matter.

Think of it this way: you can hear sound waves because they vibrate the air molecules around you. But you can hear radio waves even in space, where there's no air.