"Hmm. Maybe a necklace is the wrong choice, then," she agrees. "A better choice would be one that had literally no effect on anyone except yourself."

She thinks for a moment.

"Oh! I know," she exclaims. "I wanted to show you how to use our transport system to go places anyways. Our transport system uses windows which only you can see to let you choose your destination. And the color of those windows is different for everyone, to make it harder for someone to fake a window. What color do you want your windows to be? It doesn't affect the functionality of the interface at all, and nobody can see it except for you, so the only thing the choice of color affects is you."

"...how does it do that?" she wonders.

"How does it only show up for you?" she clarifies. "The system creates appropriate photons to make it look like the interface is there for you, aimed directly into your eyes. Any photons that are scattered instead of absorbed get deleted as they leave your eyes, for privacy."

"That's very strange," she says. "It's so... specific."

Sandalwood cocks her head. "I'm not sure what you mean? People need ways to interact with the transport system, and most people prefer visual interfaces because they are inherently non-linear in a way that is hard for audio interfaces, but permit more acuity than direct brain interfaces, or tactile interfaces, or anything like that."

"If you prefer an audio interface you can choose that instead," she offers. "I don't know how well our direct brain interfaces or tactile interfaces would work for you. But if you have some sense like magnetoception or graviception that humans lack, we can probably rig up an interface module that uses that instead. Although it didn't look like you noticed the gravity wave I tried to greet you with when you first arrived."

"...it's... making and erasing light just so, for everyone," she says. "Specifically. Isn't that... a lot of trouble?"

Someday Sandalwood will meet aliens who have graviception. She was so prepared.

"I think people sometimes have difficulty thinking about what is hard and what is easy for a computer," she begins to say, before interrupting herself. "Wait! I didn't even think to ask. Are you familiar with the concept of a computer?"

"I don't think so."

Sandalwood summons a simple half-adder made with marbles. "The basic idea is to make a physical object that can react to different things happening according to a predetermined set of rules," she explains. "Our actual computers use hybrid optronic field-tensor processors which are probably too complicated to actually get into. But it's the same basic idea as this marble contraption. If you send the same marbles through the input here, the same pattern of marbles will come through the output here. And if you think of the patterns of marbles as representing numbers in a particular way, what this machine does is produce the sum of its inputs."

She points to a label on the side of the machine showing how numbers are encoded.

"It doesn't 'understand' addition, because it's not conscious, but if you needed to sum up a big list of numbers you could create a bunch of these chained together, and they could add your numbers without needing a thinking being to do it. The hard part is coming up with a structured physical system that computes the outcome you want. Once you've done that, it takes no additional effort to use it for more problems," she continues.

"So it took a lot of time to design our current computers, and figure out how to describe the rules for when light should be created and destroyed and so on, but now that they have been designed, they can handle doing the interfaces for everyone in the solar system simultaneously with no additional effort. There is an administrator who watches over the computers in case something goes wrong -- her name is Pear -- but she normally doesn't need to step in because they're very reliable."

"...can I put marbles in it?" she asks tentatively.

"Yes, go right ahead," Sandalwood agrees, pointing at the marble storage baskets on top. "I summoned it so that you could use it to build an intuition."

She is fully prepared to summon more marble-based mechanical computers as necessary to turn this into 'alien's first computer engineering lesson' if necessary.

She picks up a marble, studies it, studies the contraption. Is it obvious where and how to put the marble in?

Yes! There two input funnels labeled "A" and "B" with little plates and colored blue. There are two output pipes colored orange and labeled "A+B 1s digit" and "A+B 2s digit". The marbles in the storage basket on top of the machine come in two varieties: a heavier kind colored white with a "1" printed on them, and a lighter black kind with "0" printed on them. The internals of the machine are made with glass and wire, so that she can follow the paths the marbles will take as they fall through.

There's also an internal reservoir of zero marbles and an extra uncolored output labeled "extra marbles" which comes out the side into a little basket.

...hmm.

She puts her 0-marble into A.

The 0-marble falls into a little basket, which tips over onto a little rail, which it almost has enough weight to tip, but a counterweight keeps it from moving, so the 0-marble rolls the other way and hits a little flipper. The flipper is connected over to a paddle in the B part of the machine that turns the other way. The 0-marble then rolls down a tube and comes out of the "A+B 2's digit" output.

...she tries putting it into B, instead.

It falls down, past a little platform that was retracted when the first basket tipped over. It hits the paddle, pushing it back to a neutral position, but not before being diverted to a different tube that makes it fall out of the "A+B 1's digit" output. Putting two marbles through the outputs also tugs on a string that tips the A input basket back upright.

Sandalwood silently watches this, and then chimes in to say. "That corresponds to the fact that 0 + 0 is 0," she comments.

Next she tries putting the 0-marble into B and then...

The 0-marble falls into the B input and then doesn't come out.

Sandalwood winces internally and attaches a note to the teaching instructions that came with the adder.

"This particular adder is designed so that it deals with the A input first, and the B input waits," she explains. She points at where the 0-marble has landed on a little platform. "If you put another marble into the A input, it will fall into that little basket and pull this platform out of the way and this marble will fall into the rest of the machine."

She takes another 0-marble and puts it into A.

It falls into the basket, tips over, and then follows the path of the first marble she put into A. It nudges the paddle before the B marble has fallen down to that point, so the B marble follows the same path too. They roll simultaneously out of the output slots, the motion resetting the basket at the top.

Sandalwood doesn't interrupt; she's pretty sure their guest is going to try a few more experiments unprompted.

This time she puts a 0-marble into A, and waits for it to come out, and then puts it back into A.

It falls down and lands on the top of the tipped-over basket and stays there, waiting for one of the outputs to pull the basket back upright.

So she puts her other 0-marble into B.

It passes the pulled-back platform, hits the redirection paddle, and falls out of the "A+B 1's place" output. This pulls the basket back upright, and the first 0-marble plops into it and then immediately tips it over again, hitting the ramp, moving the paddle back into its interception position, and then rolling out of the "A+B 2's place" output.

Sandalwood internally takes notes about how some kind of input device that only takes marbles synchronously instead of the clever little basket-based locking mechanism would probably be more robust to experimentation.