脑子 (nǎo zi): 脳は

He is getting older, and his mind is not as sharp as before; he often forgets where he puts his keys.

This student is clever; he understands after the teacher explains once and can even draw inferences.

After the car accident, his brain was severely injured, and the doctor said it would take a long time for his memory to recover.

Facing this complex math problem, he used his brain and finally came up with a clever solution.

She is under too much work pressure; her mind is tense all day, and she often suffers from insomnia at night.