"People dropped rocks on all the mammoths and made them bald?"

Honestly a very understandable confusion! Evelyn had sort of forgotten how this book contained jokes in addition to actual science lessons. 

"...I think people a very long time ago hunted and killed the mammoths - maybe by dropping rocks on them, I don't know! - and then the ice age ended and it got warmer and the climate was harder on them, and now there aren't any more. Elephants are - like bald mammoths, yeah, but they lived in different parts of the world." 

"Okay." She doesn't have any more questions yet. Inclined planes make sense.

Iomedae can't really think what it would mean for inclined planes to be pretend. They could be banned, and Americans could probably manage to convince themselves they didn't really exist, but they would, in fact, keep right on existing.

Next page: did you know that keys work off the principle of the inclined plane? Because they do!

In a standard cylinder key - (actually Evelyn isn't sure that Iomedae and Alfirin would have had locks, where they grew up? She can pull out her house keys from her handbag and show them) - but anyway, the way that works is by pushing pins out of the way the exact right amount so that the two blocks - one is the door and one is the bolt holding the door shut - can slide apart. And the reason keys have all those funny uneven serrated edges, is because that lets you push the pins to the exact right height. There's a simplified picture showing it!  

(Evelyn is finally slightly more in her element! She really likes reading science books to - not kids, in this case, but young people who know less - and being reminded, herself, about how ridiculously cool all of the little things in the world like keys are. She's happy.) 

Taldor has invented locks and keys but not being in the locks business or likely to pursue a career as a thief Iomedae never learned anything about how they work. She is attentive. 

Alfirin knows that locks exist and stop things from opening but nothing else about them.

The next page is about 'cutting machines'! ...By which the book means things like axes and scissors. Things that cut other things nearly always make use of the wedge, which is an inclined plane in the same way that ramps are! For an axe, the blade is wedge-shaped, which converts the downward force of the axe into a sideways force that pushes apart the two halves of whatever the axe is cutting. Scissors have wedge-blades that cut with great force from opposite directions, but also use levers, which will be covered on a later page apparently. 

...There's a section on electric trimmers, which sure does contain an image of someone using an electric trimmer (where "the trimmer's blades act as paired wedges like the blades of scissors") using it to, uh, shave a mammoth.

(Oh no. Evelyn really didn't plan this and now she's probably going to have to have more awkward conversations with Alfirin about why elephants are bald.) 

She already got the explanation that Americans killed all the elephants with hair, and that elephant baldness breeds true. (She is kind of confused by why they seem to care so much about elephants not having hair.)

She knows how axes work already. It's not hard to understand how axes work if you've ever seen an axe, though come to think of it she's not sure she's seen any axes in America. "Do childs in America not see axes, need books to tell about them?"

Um. 

"I had seen an axe by the time I was your age, I think."

Once that she remembers, on a trip visiting her best friend's uncle, and she didn't actually touch it or use it.

"I guess probably a lot of American children haven't, and - probably that's even more true for kids now. If you live in a city, there's not really anything you need an axe for. ...Uh, I don't think that's actually the point that the book is making, though? The point is that axes and scissors and - electric hair-trimmers - all work for the same reasons that walking up a gentle hill is easier than climbing a tower, even if it's the same height." 

Iomedae doesn't understand how that's true from Evelyn's explanation and looking at the pictures. She'll just watch Alfirin to see if it actually is true and if so memorize it to contemplate if she ever needs to.

She frowns. "The book sayed it is the same amount work to walk up a hill or to walk up a tower. I think I not know how that is for axes."

Evelyn is deeply unqualified to be a science teacher and has no idea what she's doing here. 

"I think it's - so it takes less effort to walk up a gentle hill than to climb a tower, even if you're carrying the same weight, right? And I think that's a bit like, if you imagine you were trying to cut a log into two with a square axe? You'd have to hit it over and over, really hard, and it wouldn't be very good at cutting the log, because all the hitting-force would just be going down, trying to crush it, and not sideways to split the wood? And if you had a normal sharp axe, it'd be easier. ...I think trying to climb straight up a tower with a heavy rock is like trying to cut a log with a square axe." 

Iomedae does not see how but would not expect to see how even if this was true, this is clearly a wizard kind of thing.

Something about that explanation doesn't make sense but she's not sure how to articulate it in English. "I think hitting with a square axe is like trying to go up a tower by hitting the wall?"

Man. Evelyn continues to not be a science teacher! She never claimed to be any good at explaining physics, or mechanics or whatever this chapter is. 

"I think that's not quite the same? If you're trying to climb a tower, and you hit the wall, you'll - bounce off, I guess, and fall. But if it's a square axe hitting wood, nothing can fly off anywhere? The axe can't, because it's solid metal and stuck against the wood, and the wood can't fly anywhere because it's all stuck to - itself? ...Maybe that's the difference, if you're climbing a tower and hitting it then it's only air around you and not wood." 

...She's pretty sure if you hit a block of wood with a square axe - or a hammer - the hammer will bounce off?

Evelyn is bad at explaining this, it turns out. 

"Uh. Okay, hmm. Yeah, I think it's - so you can imagine cutting wood with a sharpened axe or will a dull axe, right? And if the axe is dulled, the person has to be stronger to use it, or the wood has to be softer. I think you can say that a square axe is kind of like a really, really dull axe, and normal people aren't strong enough to hit hard enough with it for it to go through the wood, so it bounces. But if you hit, like, butter in a log shape with the square axe, it wouldn't bounce off it? It wouldn't work very well, though it'd just also squish a bunch and be messy because most of the force isn't cutting force. And if someone was really, really strong, then wood would be more like butter to them. Does that make sense?" 

That...maybe makes sense? "If someone very strong hit a tree with hammer, it break apart, not squish."

"Yeah, I think wood has more... There's a physics word for the thing I mean but I can't remember it, I think it's not 'brittle' - something brittle is usually more fragile than wood, like glass or pottery, and then you don't need to be very strong to hit it and have it break apart. Wood is - stronger than pottery but I'm blanking on the physics word for what kind of strong it is. Anyway, I think you want a sharp blade to cut something like wood in exactly the way you meant to, because you can control a lot more where the force goes? And a very strong person could break a tree apart with a square axe, but not - neatly, because the force is kind of all over the place and I think the wood would end up breaking where it was closest to having a crack already." 

Yes, wood is not like butter even for very strong people. This does not really explain why cutting wood with an axe is like walking up a hill instead of climbing a tower. "Okay."

Yeah, Evelyn suspects she didn't do a great job of explaining it, but she's more or less out of ideas now. "I bet Jeremy can do a better job of explaining it, he actually did high school physics and it wasn't as long ago." 

Next page? 

Sure, next page.

Plows! This page is pretty fun because Evelyn is learning things too. Apparently plows have four different parts, all of which have weird names that Evelyn had never heard of (Jeremy liked this book but could read to himself by then), three of which use wedges pointed in different directions to let a horse - or for modern plows, a tractor - plow hard soil with less force. 

"Steel is not expensive in America, so use it for plows!"