Question

What's on your mind 

or 

what's in your mind

which one's correct?

Answer 1

Both are true with different meanings:

 

ability to think and imagine

 [uncountable and countable] your thoughts or your ability to think, feel, and imagine things:

in somebody's mind

» There was no doubt in my mind that it was the right decision to make.

» Do you have a clear picture in your mind of what you want?

» A plan began to form in his mind.

» The event is still fresh in most people's minds.

on your/somebody's mind

a) if something is on your mind, you keep thinking or worrying about it:

» He looked as though he had something on his mind.

 (=a lot of problems to worry about)

b) if something is on your mind, that is what you are thinking about:

» She's the type of person who just says what's on her mind.

Comments

nice explain +

Answer 2

1- On one's mind(= on the brain)= In one's thoughts, preoccupying one

2-In one's mind= In the center of your thought; in your close attention