This question may seem to be very simple, but something I get confused whenever I want to speak.
I read a book entitled "re-start your English", and saw a sentence.
This is a leg. It is part of your body.
This is a neck. It is 'a' part of your body.
Why didn't they use 'a' in the first sentence? And why did they use one in the second one?
I'm not a native, so I hope you will please understand, even if there's something awkward about my question.