When I plug in headphones into my laptop the in built mic stops working. Can anyone tell me why?

[prakhg]

I have an Inspiron 3537 laptop with windows 8.1.

 

[MauveCloud]

Do you have a separate microphone jack? If not, you've probably got a headset jack that the laptop expects to have both headphones and a microphone, so it would let you use that instead of the built-in microphone.