What does 'cogs' mean in this context?

Question

He stopped there to enjoy the effect of these words. He could almost see the cogs working under Uncle Vernon's thick, dark, neatly parted hair. If he tried to stop Harry writing to Sirius, Sirius would think Harry was being mistreated. If he told Harry he couldn't go to the Quidditch World Cup, Harry would write and tell Sirius, who would know Harry was being mistreated. There was only one thing for Uncle Vernon to do. ...

Cog, as defined in dictionaries, means One of a series of teeth, as on the rim of a wheel or gear, whose engagement transmits successive motive force to a corresponding wheel or gear.

But I reckon it's used figuratively in this context. How should we understand it in this context? Are 'cogs' referring to Uncle Vernon's eyes or something else?

Answer 1

This passage compares a human brain to a clockwork-type machine. The "cogs" are "under Uncle Vernon's" "hair". In other words, the cogs are in his head.

"Mental gears" or "waiting for the gears to turn [in his head]" are more common ways of expressing this analogy.

This analogy often implies that the person thinks slowly. The person is not making the decision instantly. Instead, the person thinks just quickly enough that if you wait, the person will come to a decision while you wait.