Images taken from our memories. Like on Carcosa. [He's not eager to remind her of the theatre, but it's an apt example. He's fully aware this time, but the trade-off seems to be that she can see it, too.

Zack gets to his feet, shooting Sephiroth a look because he's totally not imaginary! (He is.)]

Sorry. I'm... [Except now Saxsice's vision is bleeding into Zack's awareness, and he does a double-take between her and the her in the apartment.] Uh, on second thought, maybe something weird is going on. Are we all on drugs or something? I don't remember agreeing to anything with the science department, but--

[Sephiroth abruptly pulls himself out of the water, and Zack, the apartment, the boy-- they all vanish. Sephiroth pauses, because he was expecting there to be more to it, but it's not his surprise that prevents him from reaching for a towel. He just has zero modesty.]

...there. They're gone. [Why isn't that more of a relief?]