This is the 14075th most frequent Korean word.
머릿속에
"In one's head/mind."
Here, "머릿속에" is used to describe that the ideas exist within his mind.
그는 항상 머릿속에 창의적인 아이디어를 가지고 있다.
He always has creative ideas in his mind.
In this sentence, "머릿속에" is employed to indicate the thoughts appearing or surfacing in one's mind.
머릿속에 떠오르는 생각을 빠르게 써 보세요.
Quickly write down the thoughts that come to your mind.
This usage of "머릿속에" signifies the location of the questions as being within one's mental space or mind.