In an optional diversion, you can explore how "Spaceship Earth" protects earthlings—both human and other animals—from many of the harmful waves that travel across the universe. This same protective quality of the ionosphere is something we exploit to send EM signals around the earth.
Show the slide that illustrates which waves traveling the universe can penetrate the atmosphere and ionosphere.
Electromagnetic waves criss-cross the universe. Many of these can easily damage our DNA and kill us. The Earth is surrounded by a protective envelope that keeps many of these harmful waves out of our livable zone. Earth's atmosphere stops most types of electromagnetic radiation from space from reaching Earth's surface. This illustration shows how far into the atmosphere different parts of the EM spectrum can go before being absorbed. Only portions of radio and visible light reach the surface.
Image Credit: STScI/JHU/NASA
Show the slide that illustrates how we use the atmosphere to send radio waves around the world by bouncing it off the ionosphere.
The protective shell that keeps some electromagnetic waves out also keeps them in. High frequency waves pass through the ionosphere and escape into space while the low frequency waves reflect off the ionosphere and essentially "skip" around the Earth to get from one place to another.
Image Credit: NASA's Radio JOVE, Solar and Planetary Radio Astronomy for Schools