Sound needs a Medium to Propagate
An electric bell is placed inside an air tight bell jar.
The jar is connected to a vacuum pump to remove air from it. Initially the jar is full of air. When the bell is switched on you hear the bell ring. Now slowly the air inside the jar is removed by the pump. As the air is removed the sound becomes fainter and fainter until there is a vacuum inside and you hear no sound at all. This experiment shows that sound needs a material medium to propagate.
This shows that thunder travels around 140 air miles to reach the ground through air medium.