"Women make very small eggs that stay inside their bodies and don't come out. Like, the size of the dot a pencil makes small."

"Huh! That sounds really...fragile? - Wait. Do chickens have the same thing, where come is what has the code from the father's side? How does that get into the egg, then? Chicken eggs have shells!" 

"My guess is that it gets in before the egg develops its shell but I have to admit I don't know much about chicken reproduction."

"Maybe the library has books about it! How about frogs. Their eggs are all squishy. ...They're not very good to eat. Not like chicken eggs." 

"Anyway. Evolution. So some people are more likely to have kids than other people, right? Like, some people die young, or are super hot so everyone wants to have sex with them, or really like babies, or do a better job of taking care of their kids, and they get more kids and more grandkids."

"- Yeah, I guess." 

"So that means that there are more of their genes in the next generation. And on average each generation has more and more of the traits that make them better at surviving and reproducing."

"Huh." 

Masozi holds very still for thirty seconds or so while he appears to concentrate intently on thinking about this. 

"And - frogs and chickens do this as well, right? Do cockroaches and beetles do it? ...Do trees and grass do it?" 

"Every living thing does this. --Bacteria are a little bit more complicated."

"....What are 'bacteria'?" 

"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."