What does 'his little grey cells' mean?

[Lam Tung]

English Language Tests, Intermediate level

ESL/EFL Test #29 "Who has done it?", question 7

One of the best known detectives is Agatha Christie's Hercule Poirot who uses what he calls his little grey ......... -- his brains.

(a) molecules
(b) cells
(c) particles
(d) atoms

English Language Tests, Intermediate level

ESL/EFL Test #29 "Who has done it?", answer 7

One of the best known detectives is Agatha Christie's Hercule Poirot who uses what he calls his little grey cells -- his brains.

Correct answer: (b) cells

Your answer was: incorrect
One of the best known detectives is Agatha Christie's Hercule Poirot who uses what he calls his little grey molecules -- his brains.
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hi!
what is mean " his little grey cells"
thanks

Lam Tung

[Yankee]

Hi Lam Tung

The sentence basically tells you what that means: his brain

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I have a question too. I assume that Alan wrote this test, so my question is for Alan:

I don't know "little grey cells" as a fixed idiom. More familiar to me would be the expression "gray matter". Is the expression "little grey cells" often used in the UK? Or is it something that is mainly associated only with Poirot?
.