Why couldn't Cobb show the totem to Mal? [duplicate]

Why didn't Cobb just spin the totem in the real world to show Mal that she is finally back to reality so she wouldn't have killed herself?

This is a great question, one that requires a somewhat lengthy explanation to answer properly but is definitely worth the read:

Firstly, a totem can only be touched or held by its creator.

Knowing what a totem's properties are -- like its weight, what it's made of, how it behaves when it's used, etc. -- must remain exclusive to its owner. When Cobb first recruits Ariadne, he and Arthur mention this:

COBB: She'll need a totem.
ARIADNE: What?
ARTHUR: Some kind of personal icon. A small object that you can always have with you, and that no one else knows.
..
ARIADNE: Like a coin?
COBB: Too common. You need something that has a weight or movement that only you know.

The purpose of this is so that if a person wants to check if they're still dreaming or not, they can use their totem to see if it behaves the way it should (i.e., how it behaves in reality). When Ariadne shows her totem to Cobb, they discuss this and she says:

ARIADNE: It's an elegant solution to keeping track of reality. Your invention?

Which leads us into part two -- that the spinning top was originally Mal's.

In response to Ariadne's question, Cobb responds by saying:

COBB: No. Mal's. This one was hers. She'd spin it in a dream and it would never topple. Just spin and spin...

So it was only after Mal killed herself that Cobb took the spinning top to use as his own totem. With this in mind, in order for Cobb to use the spinning top as a valid argument that Mal was no longer dreaming, Cobb would have needed to convince Mal to spin the top herself; he wouldn't be allowed to do it.

So then, why didn't she just spin it herself?

When Cobb explains to Ariadne what happened to Mal, he mentions that at first Mal seemed alright but then she started acting differently and eventually she told him what had been troubling her; that she thought they were still dreaming.

COBB: At first Mal seemed okay. But I started to realize something was wrong. Finally she admitted it. This idea she was possessed by. This simple little idea that changed everything...
ARIADNE: What was it?
COBB: That our world was not real. No matter what I did, no matter what I said, she was convinced that we were still in a dream. That we needed to wake up again...

Mal was plagued with that idea because that's how Cobb convinced her to leave the dream world they had created, he himself had performed Inception on her.

Now at this point, it could have gone two ways:

1. Since Mal was already completely convinced that they were still dreaming, even if Cobb did plead for her to spin the top she would have said no, that there's no point in doing so because she already knows the truth.

Or,

2. Mal actually did spin the top at some point but still wasn't convinced. At one point in the film Cobb describes what Inception is like:

   COBB: An idea is like a virus. Resilient. Highly contagious. The smallest seed of an idea can grow. It can grow to define or destroy you.

   I mean, we do see her playing with the totem while contemplating her reality so there's a good chance she spun it when Inception first started taking a strong hold on her.

   

In my opinion, I believe that Mal did spin the top at some point but it just didn't convince her, Inception was too strong.

One critical aspect about this plot device that's definitely worth mentioning and that's kind of always bothered me over the years when watching this film: there's an underlying assumption that the totem will always behave abnormally in a dream than to how it does in reality. Though it's never mentioned, perhaps the dreamer also attempts to make the totem behave in a way that defies physics; for example -- Mal/Cobb's totem spinning indefinitely and Ariadne's chess piece [possibly] bouncing back up or indefinitely bouncing around once knocked over (she's never shown using it so we don't know how it'd behave in a dream). So really, in order for a totem to be fully functional and reliable its creator also needs to determine how it should behave when within a dream, otherwise there stands a chance that it'll behave perfectly normal and the dreamer won't know one way or the other.