Both "What's in your mind?" and "What's on your mind?" are commonly used phrases to inquire about someone's thoughts or what they are currently thinking about. While they are similar in meaning, they have slight differences in emphasis and usage.
"What's on your mind?" is the more common and idiomatic phrase used to ask someone what they are thinking about or what is occupying their thoughts at the moment. It is widely used in everyday conversations and is considered more natural in this context.
Example: "You seem preoccupied. What's on your mind?"
"On your mind" here means the thoughts or concerns that are currently occupying the person's thinking.
"What's in your mind?" is also grammatically correct, but it is less commonly used in this context. It might be interpreted slightly differently, focusing more on the contents of someone's mind or what thoughts they might be keeping inside, possibly implying a deeper level of introspection.
Example: "You look deep in thought. What's in your mind?"
Both phrases can be used to inquire about someone's thoughts, and either would be understood, but "What's on your mind?" is more typical for casual conversations about what someone is currently thinking.
If you want to ask someone about their thoughts in a more common and straightforward way, "What's on your mind?" would be the better choice.