How would you explain it?
Sound waves propogate through vibrating particles - the air, water, metal are made of particles, and sound waves move through the mediums as one vibrating particle pushes against the next. Radio waves, on the other hand, are a form of electromagnetic radiation, the same as light, X-rays, ultraviolet etc. Totally different. They are made of electromagnetic fields, rather than vibrating particles, so light can travel through a vacuum, hence we can see the sun and the stars.
Not bad. But what if the caller then said, "Well, if it's full of electromagnetic fields, it's not a vacuum, is it?"
I realised how unknowledgable Dr. Karl can be yesterday; he's in the Physics department at Sydney Uni isn't he? Well yesterday he couldn't adequately explain to a caller how sound waves can't travel through a vacuum, but radio waves can. Just thought I'd mention that.