"Uh, living things too small to see that sometimes make you sick."

"- Oh! Is that the thing that maybe happened to me on Monday, if it wasn't someone trying to poison me? ....I think it probably wasn't or they'd've tried something else when it didn't work, right?" 

"Maybe they're biding their time? You've spent a lot of time in your room."

"Maybe. ...How long d'you think it'll have to be before we can say they're not gonna murder me? I - I don't want to spend the next year being scared and having to hide in my room most of the time!" 

"I... think probably that's not the kind of thing I can have opinions on because I'm reckless or whatever."

....Yeah that's probably true, and Masozi is kind of grateful to Wei Wuxian for spontaneously bringing it up. 

"Anyway. Evolution. So - it's just a math thing, right, if some people - or animals - have more children, that's more of their code that exists, and so you do that over and over and over, and - eventually you end up with code that's better for surviving and having children?" 

"Yep! And sometimes the code isn't copied exactly right-- so maybe your body makes a mistake in copying and it tells your kids to have lighter skin-- and then if lighter skin helps your kids survive and have their own children it spreads throughout the population. And that's why people have different skin colors, some people's bodies made a mistake in copying the code for skin color and dark skin is better for surviving in Africa and light skin is better for surviving in China."

"Huh, why?" 

"Your skin needs to absorb sunlight to make a particular chemical but if you absorb too much sunlight it can mess up the code and in particular the part of it that tells your skin 'stop growing more skin, we have enough skin.' And you get more sunlight in Africa than in China. --Uh, I feel like the way I explained it sounds weird, it creates tumors? I don't know if you know the word 'tumor.'"

"Ooh. I - didn't know that word but I think I know the thing you mean, it's when people get lumps and eventually they get sick and die - it doesn't just grow on skin, right? We knew a lady once who had one in her breast and eventually it started leaking horrible pus and then she died." 

"Yeah, that thing-- the code of any part of your body can get messed up but the sun only messes up the code on your skin, I think, because it doesn't get in to your organs."

Nod. "Right. But if you have pale skin in Africa then there's - you get more tumors or your kids do, and getting more of the chemical doesn't help enough to account for that, and so eventually all those people die and don't have kids and then everyone who's left only has dark skin? And.... Is everything like that? Like - having legs, or having eyes - except I dunno, that seems more complicated? Maybe you can get light skin from the code being copied with a mistake but I don't see how you could get legs from that." 

"Well, it takes millions of years. But eyes start out as something that can just sense light, and then they get a little better, and then a little better, and then you get full eyes. --I can go into more detail but that involves a major tangent about how eyes work and I think it's hard to explain if you don't have an eye to dissect so you can look at the parts."

"I cut up a dead fish I found once!" Masozi offers brightly. "I could draw a picture of what its eye was like. ...Did fish and people both have to start with no eyes and get all the way to proper eyes?" 

"Oh, no! Fish and people are related, the common ancestor of fish and people developed eyes."

"I - wait what? How does that work? Fish and people have babies in totally different ways and - and breathe in different ways - how can you have something that had babies and then some of them ended up changing over time into fish and some of them turned into people????" 

"If you have a bunch of small changes they become really big changes. It's not a big change to evolve lungs that can breathe air a little bit, and then they get a little better, and then a little better, and there's lots of food and no predators on land at that point so it makes sense for some animals to evolve to be on land, and then their fins slowly change to be better at walking on land, and after millions of years you get people."

(They arrive at the reading room.)

"So...the ancestor of fish and people started out more like a fish? What was before that?" 

"The very first life was one-celled organisms-- a cell is one of the smallest units that make up your body, I can draw a picture of one if you want-- and then after that was algae I think?"

Masozi doesn't know the word 'algae' and looks blank. 

"Uh, the green stuff on top of lakes?"

"Oh, that? ...Do you mean the kind that has long stems and floaty leaves, or the kind that's like grass with bubbles, or the kind that's sort of like threads tangled together, or the kind that's gooey like mud?" 

"Threads tangled together, I think. --Anyway, evolution is why everyone finds sex so interesting, people who wanted to do the thing that makes babies a lot ended up making more babies."

"....Right. I guess that makes sense. But - why kissing, then? That's not related to making babies, and lots of times people kiss and then don't fuck so it's not part of sex, it's just...a different thing that people find interesting for some reason?" 

"Mouths have a lot of nerves and people tend to like touching parts of their bodies that have a lot of nerves to other people's bodies, especially if they're attracted to them. I'm not sure why that evolved? Maybe as a way to make relationships stronger since human babies take a lot of care so you have to make sure people like each other if they're going to have one, so evolution was like 'here's a sex-associated thing that is also fun and makes you like people.'"