Body part with billions of neurons (5)
I believe the answer is:
brain
(Other definitions for brain that I've seen before include "The grey matter" , "Whack on the head" , "Thinking thing" , "Clever person" , "It's in the skull" .)
I believe the answer is:
brain
(Other definitions for brain that I've seen before include "The grey matter" , "Whack on the head" , "Thinking thing" , "Clever person" , "It's in the skull" .)