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Phantasmal Force can be used to place a delusion inside a creature's head for the duration. I understand that the spell is worded very openly to encourage creative usage, but I'm not sure about how I should be ruling it case-by-case. I understand the purpose of most other illusion spells, but I just can't seem to get my head around certain aspects of this particular one.

The target can use its action to examine the phantasm with an Intelligence (Investigation) check against your spell save DC. If the check succeeds, the target realizes that the phantasm is an illusion, and the spell ends. While a target is affected by the spell, the target treats the phantasm as if it were real. The target rationalizes any illogical outcomes from interacting with the phantasm. For example, a target attempting to walk across a phantasmal bridge that spans a chasm falls once it steps onto the bridge. (PHB pg.264)

According to the spell's description, the target would rationalize any illogical outcomes. If this is the case, why would he ever choose to investigate it if every illogical outcome seems perfectly reasonable him while? Would someone telling him he isn't being assaulted by a fire elemental prompt an investigation check, or would this count as "Interacting with the phantasm"? Are there any other activities that could prompt an investigation check?

Question: When would a creature use an investigation check on an illusion created by Phantasmal Force?

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