on/in your mind

What's the difference between "on your mind" and "in your mind".  Do we use "What's on your mind" for question like this?

What's on your mind means, what are you thinking about. What's in your mind. Isn't a sentence. Well,  techincally it is, but it doesn't make sense. 

Yes, you would use what's on your mind. 

"on" is used or should be used when your talking about it.

"in" is used when you're referring to something inside of it. 

I would say the correct answer is what's in your mind which means what's inside of your mind (your thoughts), what's on your mind means what's your mind made of (brain, blood, oxygen).

^^ that's incorrect, it would be the other way around. When you wanna know what someone is thinking you ask them "whats on their mind" not what's in there mind.

"on  one's mind" and "in one's mind" are both all right, I am  sure of this! The former one means "get somebody worry about something" ,The latter one means "what's somebody thinking about",as follows are two examples"

Your little brother is ofen on your mind!(你总是替你弟弟操心)

I suggest going to kunMing for travelling durning National Day,What's in your mind?(我建议国庆节去昆明玩,你的意思呢?)