in/on you mind

What's on your mind? means someone wants to know what you're thinking. As in, I come to you and say: "yoong, may I talk to you? " And you say, "why sure, what's on your mind?"

What's in your mind? is pretty much asking the contents of someone's brain and not likely to come up unless you are a brain surgeon.

Thanks, Gala.

Maybe it's different in UK English, but if something is on your mind it often means that it is preoccupying or worrying you.

I met Mary last week and now she is always on my mind!

If something is in your mind it often means that it is an intention.

It is in my mind to ask John to dinner.

Mother told me what was in her mind concerning our holiday destination.

Thanks, Contrex.

Also, if something is in somebody's mind, it can just be a way of saying that they are thinking of, or are aware of a fact or possibility. As always, context is all important.

Once again, thanks, Contrex.