Directly asking:
* "What's on your mind?" - This is a straightforward and common way to ask what someone is thinking.
* "What are you thinking about?" - Similar to the previous idiom, this asks directly about the contents of someone's thoughts.
More subtly:
* "What's going on in that head of yours?" - This is more playful and implies curiosity about the person's thoughts.
* "You seem deep in thought. What's up?" - This idiom acknowledges the person's contemplative state and invites them to share their thoughts.
Figuratively:
* "Let's get inside your head." - This idiom is used to describe a desire to understand someone's perspective or thought processes.
* "I can't read your mind." - This idiom is used to express frustration or confusion about someone's thoughts and motivations.
The best idiom to use will depend on the situation and the relationship between the speaker and the person they're asking.