Grey matter

"I'd better be putting my grey matter into that algebra instead of wasting it plotting for a party dress that I certainly can't get."

"Inspector Pepys!" gasped Bernard Megger, swallowing between the words, "I shall remember you!" "You will be wasting grey matter!" replied the man.

How many phone numbers are stored in your grey matter?

He fell from the 5th floor. There is grey matter on the path behind his head. I'm afraid he's dead.