The doctor tells you to sit quiely while he listens through his stethoscope.
You are quiet - or so you think.
On the inside you are not quiet at all.
There's a "lub-dup,lub-dup" sound.
The "lub-dup" sound is your heartbeat.
Place your hand over your chest and you can feel the beat.
Before the stethoscope was invented,doctors listened to the heartbeat by placeing one ear against the patient's chest.
It can pick up sound from different parts of your chest.
The doctor can hear the sound of each moving part of your beating hart.
Instead of a mixture of sounds,the doctor gets the sounds of different parts picked out by the end of the stethoscope.
The rubber tubes carrythe sound to his ears.