Phaneron

Answer: When Cobb used inception on Mal, the idea that her world isn't real and she needed to kill herself took over her mind. Mal made an error in telling Cobb how her totem worked before they ever went into limbo. Cobb was able to use her totem specifically to implant the idea in her head. Since he knows how her totem works, it can't work as a totem anymore. Spinning the top in front of her and having it fall over would not prove that she wasn't dreaming anymore. Ironically, Cobb makes the exact same mistake as he continues to use the totem even after he tells Ariadne how it works. As an important note, the spinning top itself is a poor choice for a totem, since the thing that makes it special (it never stops spinning in the dream world) can be easily observed by anyone else. The entire point is to have you, and only you, know what makes the totem special. Arthur's loaded die is a better choice because only he notices how the weight is uneven in his hand. Ariadne's totem is not described but it seems to have similar properties to Arthur's in that she has altered its balance in a specific way.

BaconIsMyBFF

Answer: The movie implies that all the time spent in limbo made her lose her mind, so if Cobb tried to demonstrate the totem proofs for her, she might have dismissed the evidence anyway.

Phaneron ★