is this a correct expression? or to be more exact, is it appropriate expression?

coco lefertile brain,

Not fertile brain.

A fertile mind is OK.

How do you decide whether the collocation (in this case, adj+n) works well (idiomatic)? I googled "fertile brain" and "fertile mind". Results showed that both of them are used on internet.

Does "ok" mean it is grammatically correct, but not often-used? what's an appropriate substitute for a fertile mind?

The brain is the organ composed of nerve tissue in the skull. It is a medical term. It is physical. When you write "fertile brain" is is like the fertile land - good for growing plants. Growing things in your brain is rather odd.

The mind represents the functional thought processes of the brain. The "fertile mind" is allegorical, not physical. It means that you are receptiv

I disagree. "Brain" is often used figuratively for "mind." Compare to the plural usage "He's got brains." The Ngram server finds "fertile brain" used almost as frequently as "fertile mind." ("Fertile imagination" beats them both.)

Curiouser and curiouser. I'm not at all familiar with "fertile brain". Maybe I need to get out more.

CJ

P.S. A

I suppose it's possible. I've seen pictures of folks with fertile brains, like these:

futile, febrile, whatever. (I think an emoticon goes here, but I'm too old to know for sure.)

I don't even have the excuse of playing American football.

I guess you are making fun of yourself? I know it is dull if a pun or any play with words is explained. However, would you mind explaining "Are you sure that wasn't "turtle brain"? " ,together with "Maybe I need to get out more" ? I am afraid that I may have misunderstood you.

coco le However, would you mind explaining "Are you sure that wasn't "turtle brain"? "

"turtle" rhymes with "fertile" but it would have the opposite meaning with "brain" because turtles' brains are very small. It was intended to be a mildly amusing break from the serious talk of the forum.

coco leMaybe I need to get out more