That and unrealistic fantasies, like Okuu's nuclear fusion reactor. Basically anything that falls into obscurity, y'know, beyond the "border of common sense". And thing is, line between than and modern/relevant stuff is pretty fragile. An old mobile phone and (in retconned CoLA chapter) a handheld console are mentioned as objects that fell into Gensokyo. I feel that's what Yukari means in that "cruel, cruel thing" quote.

So that means, if something was the cutting edge of technology in the past (for example: PlayStation 3 in 2006) that's now seen as an old thing of the past, it will then disappear from the real world and be in Gensokyo, more specifically; in Muenzuka and/or the Road of Reconsideration. Am I getting what you're saying?

Youmu:

> "Is she even still around?"

Myon:

> Keep taking in water, with the goal of filling our current Youmu-shape to the top.

> "Yeah, she went into one of the other rooms. I think she's filling out her notes on what's happened today." Reimu comments.

> You keep going, though you notice the turtle has dove down to inspect you.
> You're not halfway full even, but it doesn't look too happy with your work.
>_

Youmu:

> "It said to 'open at twilight'. In a really strange, weird-to-read way."

Myon:

> If we're at the pond, dip into it, fully submerging ourselves and standing on the bottom, keeping 'solid' the whole time.
> Then take water into ourselves.

> Marisa thinks for a bit.
> "Does that mean we have to wait for the sunset, or can we just wave a light across it constantly and trick it?" Marisa questions.
> "Um... Before... that happened..." Alice speaks up. "We were going to say that Rinnosuke has the ribbon now, and it doesn't seem like much negative effect happens to him. At least, in what he's explained. Unless Aya already came by and told about such."
> "She did." Marisa responds.

> You head into the pond, the turtle moving aside slightly, as if to make way for you, despite you being a phantom...
> You start to "carry" the water. It feels a bit weird, notably different and less pleasant than carrying something more physical in yourself...
>_

Youmu:

> Close the box, quickly!
> "Uh..."
> Frown a bit.
> "...if you're gong to have instructions on when to open the box, why would you put them inside the box?"

> You close the box, and it locks on it's own with the key still in it's lock. Something feels off about the key when such happens, even though you see no flash from it.
> "That's pretty counterintuitive..." Marisa comments. "Seems like whatever was happening failed though. Was it supposed to open like that?"
> "Gone as fast as it showed up." Reimu approaches. "What was that?"
> "Hey, if it's from around your shrine, maybe it's your god?"
> "I don't think the Hakurei God would be locked away in a box. But that felt pretty strong..."
> "Mm... you'd think it'd be locked in an orb or something spherical given the track record, huh?"
>_

Myon:

> Head out of the shrine, passing through walls as needed, in the direction of the nearby body of water.

Youmu:

> "Wait. Marisa, can you walk by thy box again?"
> If the box glows again, try opening it while it glows.

> You head for the pond you saw.
> It seems fairly well taken care of, there's even a tortoise in it. Or, maybe it's just a big turtle?

> Marisa does so, wondering what you're getting at.
> You twist the key in the moments it glows, and the box opens up, releasing some hot smoke.
> Though, the smoke soon dissipates...
> You can see something inscribed inside the box, though aside from it, nothing else appears to be present.
> It reads: Open in Twilight. The text feels a bit weird to even look at.
> "What the hell was that?!" Reimu jumps up while you're in the middle of reading it.
> "Was that a spell?" Marisa covers her mouth. "It looks like it went bad or something."
> "What was that magic?" Alice questions, seemingly arriving with Miko once more.
> Aya doesn't chime in. You wonder if she's busy with something elsewhere in the Shrine, or if she just left.
>_