Diana speaks to Steve:
Diana: It was just a coffee cup...and I lost it.
Why did Diana compare Themyscira to a coffee cup?
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In short, she doesn't. She's describing the process of making something invisible, something that her father did with Themyscira, and she's stating that she's only done that process once, with a coffee cup.
Diana: My father hid Themyscira from the world, and I’ve been trying to learn how he did it.
Steve: Did what?
Diana: Made something invisible.
But in 50 years, I’ve only done it once.
Steve: Well, now’s not a bad time to start trying.
How long does it last?
Diana: I don’t know.
It was just a coffee cup… and I lost it.