What Causes the ‘Doppler Effect’ in Sound?

Imagine you're on a playground, and a friend is riding a bike while ringing a bell. When they come toward you, the sound of the bell seems to get higher, like it's shouting at you. But when they ride away, the sound gets lower, almost like it’s whispering. This change in pitch happens because of the Doppler Effect. It’s all about how waves move and how we hear them.