"I think so," she says cautiously.

"Okay. The next insight came from a scientist doing experiments with metals and acids, but it's easier to show this using electricity," she continues.

She summons a tank of water with two electrified wires stuck into it at opposite ends, and protruding tubes to catch the Hydrogen and Oxygen this produces.

"When you run electricity through water, it breaks up into two different colorless gasses," she narrates, indicating the slowly-filling tubes. "These gasses behave in very different ways, though."

She lights a tea-light in a beaker, and then picks up the jar from the scale.

"When you take some heavy air from something that was burned in a closed container, and you pour it over another flame, it goes out," she explains, unscrewing the lid of the jar and pouring a bit of carbon dioxide over the tea-light, smothering it. "If you then introduce more of this one of the colorless gasses, though, the flame comes back," she continues. She attaches a tube to the place where the oxygen is collecting, and pours some into the beaker, making the tea-light flare back to life.

"Why do you think that might be?" she asks. "I'm not expecting you to know, I just want to hear some theories you think fit this evidence, so I know what else I need to rule out using a different demonstration to prove what's really happening to you."

"I'm not sure!" she says. "I don't know if I know anything about how fires work. I think... they go out if you put water on them, or sand... or cover them... wait, you made that heavy air by burning something, that feels important. That means... that fires make something that hurts them? But wait, if they make something that hurts them, how do they manage not to put themselves out immediately? What's different? ...if you pour the heavy air on the candle without the glass around it, does it still go out?"

"That's a great question!" Sandalwood replies. She summons a new lit tea-light sitting on its own on the table, and then pours a little more carbon dioxide over it, putting it out. She has to pour a bit more of it to smother the candle for long enough that it doesn't re-light.

"So yes, pouring heavy air over a fire can put it out even if it's not surrounded by glass," she states. "I put the tea-light in a cup to begin with because it's a little easier to do the demonstration that way. The cup catches the heavy air and you don't need to pour it as precisely. But the cup isn't necessary."

"As for what's different about it, let me show you a demonstration that old scientists couldn't have done. I'm going to make the critical component of the heavy air that I'm trying to work around to glow purple, so that you can see it, even though it's normally colorless," she says, suiting action to words.

She re-lights the tea-light, and a streamer of purple gas immediately rises off of it and up into the atmosphere.

"When a fire burns normally, the heavy air it creates is hot -- heated by the fire. This makes it rise into the air and disperse."

She summons another cup and sets it over the tea-light.

"If you trap the heavy air, though, it builds up and can smother the flame."

The tea-light winks out.

"And then with the fire gone, the heavy air cools down, and you can collect it and pour it and so on," she finishes, waving her jar of heavy air (now purple) demonstratively.

"So with that in mind, we're pretty close to figuring out what's going on here. You have a fire. It burns, producing heavy air. If a fire is surrounded by heavy air instead of normal air, it goes out. If you take a hot thing that would catch fire, but can't because it's surrounded by heavy air, and you pour this water-gas on it, it catches alight. What might explain that?"

She considers.

"...can you make the water-gas glow a colour?"

She smiles. "It might undermine my point a little to just show you, but sure!"

She snaps her fingers, and all of the (oxygen in the) visible air around them starts to glow blue, making the terrain beyond a few meters grow hazy.

She moves closer, so she can see more clearly. It's in the air all around them, but there's more of it in the jar of specifically water-gas. And... "Show me the fire, and the pouring heavy air on it?"

She ensures the tea-light is lit, and then takes the jar of heavy air and pours a streamer of it over the flame. With the color overlay, it looks like a purple liquid pours over the wick, shoving the light blue air out of the way. The flame sputters and dies, even as the purple air starts to slowly mix with the blue and blur out into an indistinct haze.

"And when you put the water-gas on it..."

She pours a measure of water-gas over it, dispersing the heavy air and bringing the (still hot) wick back to life. It struggles for a moment, but then catches and starts converting the water-gas to heavy air, sending a growing streamer of hot heavy air upwards.

She nods, satisfied.

"So the fire needs the water-gas, which it normally gets from regular air, but the heavy air pushes the water-gas out of the way."

"Yes," Sandalwood agrees. "Exactly so. Well done! That's also why putting out fires with sand works -- it blocks their access to the air, and they can't get water-gas."

She dismisses the spent candles to make space, and takes the other gas she's been extracting from the water and colors it yellow.

"So what do we know about water-gas? It's present in the air all around us, you can get it from water, and fires need it. But there's another two facts that I want to show you about it."

She connects the tubes of yellow glass and blue glass to a spherical glass reaction chamber, and lets them both rush in. They combine in a rush of pale flame. She keeps the glass of the reaction chamber cool, and the steam inside starts to condense and run down to the bottom.

"Firstly, if you combine it with the other water-gas, they turn back into water."

Next she summons a sealed bell jar with a tomato plant in it, filled with heavy air.

"I'm going to speed up time in this jar for the demonstration because otherwise it would take a while," she warns. She snaps her fingers, and the tomato plant grows, converting the heavy air back into water-gas.

"Secondly, plants can turn heavy air back into water-gas," she continues.

"So we know there are multiple ways to produce water-gas -- from water or from plants. And there are multiple ways to turn it into something else, either back into water or by letting a plant grow. You could just memorize all these facts as just a bunch of rules, but these are just the basic rules of water-gas, and there are thousands of possible chemicals. Can you think of a single idea which explains all of these facts about water-gas so that you don't need to memorize them individually?" she challenges. "Go ahead and ask for more demonstrations if you think they will help you figure it out."

"—there was something like fire, but it didn't make the purple air," she says. "When the yellow and the blue made water. Um, that's not the answer to your question, sorry."

"No, that's a good observation!" she replies. "That's another hint to the deeper idea, actually."

She sets up a bunsen burner with a constant stream of yellow water-gas, and then sparks it. The result is a pale, nearly invisible flame that pulls in nearby blue water-gas, but produces a faint ribbon of steam instead of heavy air. She puts a piece of glass over the flame, which fogs up slightly from the steam to make it more visible. The entire setup is difficult to see, but produces a noticeable heat.

"Here it is happening continuously, instead of just once," she says. "What do you notice?"

"Hmm..."

She studies the process.

"The yellow air comes in, and the blue air comes in too, and they make... steam, not just water... because fire is hot and so the water comes out hot?"

"Yup!" she agrees. "That's completely correct. So really, we need to amend our facts about blue water-gas: when burned with a candle, it makes heavy air. When burned with yellow water-gas, it makes water."

She produces a sealed flask filled with very fine iron powder and pure blue water-gas.

"We can burn it with other things, too. This is powdered iron," she explains. She shakes the flask to distribute the dust and then holds it in the hydrogen flame to heat it. After a moment, there's a whump noise, and the powdered iron converts to red iron oxide with a pulse of orange-brown flame.

She opens the flask and pours some of the iron oxide out into her hand. "And this is rust. Normally, rusting happens more slowly, but the small size of the powder, the heat of the flame, and the pure blue water-gas make it happen more quickly. I can do a demonstration that turns the rust back into iron using yellow water-gas if you want to see that, but that is another one that takes an elaborate setup and a long time."

"So what do these transformations have in common?"

"...hmm. They're all making one thing into another... do they all make the same weight afterward that they had before, like you were saying that fire does?"

She nods. "They do. I can repeat the demonstrations on a scale if you want proof, but every transformation I have shown here maintains an equal weight."

Technically some of them release a tiny bit of mass as radiant energy. But her statement is true in a 'lies to children' sense -- adding the caveat now would make their visitor more confused, not less.

"...hmm. You said you can turn the rust back into iron," she muses. "And you got the blue and yellow air by making them out of water, and then they go back together to make water again. And the heavy air... hmm, it came from a candle, but then you put it into a plant, which isn't really the same thing... no, wait, did it come from wood originally? I don't remember for sure."

"You're on the right track," she responds. "And the answer is that you can get heavy air from burning either wood or a candle. Candles are actually very similar to wood in a way that will make sense once you've figured this out."

"I am much less sure than you are that I will figure this out so it makes sense to me," she says with faint amusement. "But okay. You can turn one thing into another in a way that doesn't change how much of it there is, and you can turn things back from what you turned them into... so... hmm, what if you turned things back all the way? What's the first thing that a thing ever was? Do you know?"

"Oh! That's actually a really interesting question!" she remarks enthusiastically. "Hmm. How best to explain ..."

"So there is a different type of transformation that uses different rules -- among other things, it can change the weight of something -- which only happens naturally inside of stars. And the actual answer to your question is that everything used to be yellow water-gas, and stars turned it into other things. But that doesn't help with trying to build an intuition for this concept. If you ignore star transformations, the first substances were these 94 things," she continues, stepping over to the periodic table and indicating everything before Americium.

She takes the opportunity to point out yellow water-gas (1st on the table, labeled Hydrogen), and blue water-gas (8th on the table, labeled Oxygen). Heavy air doesn't appear on the table.

She looks over the table very carefully. The yellow air is properly marked yellow, and the blue air is properly marked blue, but, "There isn't any purple air here. So... purple air was made out of some of these things? Which ones?"

Sandalwood is momentarily conflicted, because she was trying to justify the assertion that things are made of other things, and that elements don't change or get destroyed, not just list facts. Answering questions is still probably more helpful than trying to continue using the Socratic method, though.

She points to Carbon (#6) and Oxygen.

"Purple air -- usually called carbon dioxide -- is made from one part carbon and two parts oxygen," she replies. "The thing that wood and candles both have in common that I alluded to is that they both have lots of carbon in them. They're not completely made of carbon, which is actually where ash and soot come from. When you burn wood, the carbon and the oxygen in the air make carbon dioxide, and the non-carbon parts of the wood are left behind to make ash, or bond with oxygen in their own way and make soot. Wood also has Nitrogen (#7) in it, which turns into a different colorless gas when its carbon is taken away. I didn't bother giving it a color because it wasn't relevant to the point I was trying to make with the demonstrations."

"...I see," she says. "So... the purple air shouldn't really be purple. It should be blue and, I guess, red, for carbon? But if every thing that's one of the first 94 things has its own colour that's 94 colours. I think it would get confusing and hard to make out. Especially if—you said purple air is one part carbon and two parts oxygen, but can they combine in other proportions, and do they act differently when they do? Because if so, then each thing would need its own pattern of the colours of what it's made of, and that would get really confusing..."