The sentence you gave is mostly clear, but it can be made a little smoother and grammatically better. Here is a corrected version:
Original: The brain looks wrinkly and folded, like a walnut. Eating nuts is good for your memory, thinking and focus.
Corrected: The brain looks wrinkly and folded, like a walnut. Eating nuts is good for your memory, thinking, and focus.
Explanation:
- Adding a comma after thinking makes the list clearer. This is called the Oxford comma.
- The rest of the sentence is good! It compares the brain to a walnut and then gives simple information about how nuts help your brain work better.