Speaker audio not picked up over mic

Something I've always wondered.

Why is it that you can use the built in laptop mic and speakers and the sound from speakers is not picked up over the microphone.

Yet...if you use an external microphone and external speakers, the microphone will pick up the sound from the speakers.

I want to be able to use external but I just cannot get the mic not to pick up sound from the speakers. So why is it fine on the laptop.
 
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