system
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human
[Inception] How does Cobb's totem work?

In the Inception universe, infiltrators can break into your dreams at any minute. However, Mal Cobb came up with a fairly eloquent way to test if you are in someone else's dream: an ordinary object that is modified in a way that a dreamer wouldn't know to replicate. 

This can be exemplified in the opening scene, when Saito realizes he must be in a dream because the carpet feels wrong. Even though the architect had perfectly recreated the apartment visually, he didn't know how to replicate the feel of the carpet. 

Which brings me to the problem with Mal/Cobb's totem. It's supposed to be an object that a dreamer expects to behave in the "normal" way, but the owner knows to behave abnormally. Any dreamer would assume a top would fall over; why would the top keep spinning in someone else's dream?
gpt
Firstly, you're slightly mistaken about the purpose of totems.  They aren't to determine if you are dreaming or not, they are to determine if you are in *someone else's dream*.  Arthur has his die which will always fall a certain way.  If he makes the dream, he will be able to predict it's behavior because he made it.  If someone else is attempting to extract from him, or has trapped him in an architectured dream for some reason, they won't be able to correctly make that die because they won't know how it works.  Likewise Arriadne's bishop falls in a peculiar way.  It will fall that way whether she is in reality or in a dream she architected.  However, if someone like Dom were to bring her into his dream, he wouldn't be able to replicate it (because she didn't let him touch it) and thus she'd know she was in his dream.  
That said, Dom Cobb is batshit insane enough to need more than that.  He needs a way to know if he is dreaming or in reality.  Everyone else knows this implicitly.  Think about it, when was the last time you were awake but thought you were dreaming?  That doesn't happen.  The reverse does of course, but if you're thinking to check your totem you're already past that point.  So what Cobb has is a way to check if he's dreaming or not.  He does this by attempting to will the top to keep spinning.  He also has a wedding ring that he will into existence on his finger in dreams that he doesn't wear in reality as a backup measure.