The resounding sound of an impact against the castle walls drew Twilight’s attention. She sighed and got up from her readings, heading over to the window. She stuck her head out and saw Rainbow Dash attempting to peel herself off the solid exterior wall.

“You know, this isn’t the library,” said Twilight.

“Ugh, tell me about it.” Rainbow shook the stars out of her vision and barged through the window. “You should put up some branches and have the walls converted to wood. Did I at least make a cool crash?”

“You didn’t crash, you splatted.”

“Oh come on, it wasn’t that bad,” Rainbow protested.

“When a pegasus hits a castle, that’s what happens,” Twilight replied. “It’s physics.”

“Yeah, but I'm no ordinary pegasus! I bet physics can’t explain me!”

Twilight grinned slyly. “Oh really? Want to prove that?”

Rainbow puffed up her chest. “Sure. Any challenge you can throw at me.”

“Well then, Rainbow. I challenge you to stay awake through the physics lesson that I’m about to give you.”

It was about that time that Rainbow realized that slamming into the castle had been a really bad idea. She was a slow learner.

Before she could protest, Twilight whisked her off to the castle study. It was crammed with books and whiteboards. Twilight found some markers and a clean space to write. “Okay, let’s begin. A body in motion tends to stay in motion unless acted upon by an outside force. What happens when a pegasus hits a wall?”

“Well if it’s me, I punch right through it!”

“Didn’t you just splat against my castle?”

“That wasn’t a splat, that was a crash!”

“You’re a meatbag hitting a stone wall. It was a splat. The wall exerted an outside force, and your body in motion came to a sudden halt.”

“Well…” A lightbulb finally clicked on in Rainbow’s mind. “All I’d have to do is go really fast. If I hit it hard enough, I’d punch right through.”

“Even if you did, then you’d die with a splat. A lot less dignified than going out with a bang, I’d say.”

“Well, I took out Applejack’s old barn with a bang.”

“That was a rickety wooden building that you’d already weakened. You couldn’t do the same to a sturdy stone wall.”

Maybe Twilight was right. There were a lot of things Rainbow could do, but she’d never tried punching a hole in a castle wall before. But it sounded awesome.

“Then teach me. I want to bang things.”

Twilight’s eyebrows went up and she concealed a grin. “Are you sure? You’re asking me to teach you?”

“If anyone knows physics, it’s you, Twilight! I’ve got get busy and learn how to punch holes in things!”

“If you really want to know about penetration, then I think I can help you.” Twilight picked up a marker. “Let’s start with force. Your squishy body applying a force to a sturdy castle wall, for example.”

“I’m not squishy! Do you even know how much I work out?” Rainbow flexed.

“You’re a lot softer than a castle. But that means as your body deforms while splatting on the stone, you’re transferring all your force to the wall. You aren’t bouncing off.”

“What’s the point?”

“My point is that if you want to apply maximum force, keep squishing and do less ricocheting.”

Twilight drew a simple diagram of a pony splatting into a wall. “What do you think happens when all that force hits the wall? Obviously, you didn’t punch through.”

Rainbow shrugged. “I don’t know. Maybe a picture falls off the other side or something.”

“Could be. A body slamming into a wall and causing damage to the inside of the wall is called spalling.” Twilight added a few cracks in the wall to her drawing. She turned back to Rainbow. “But what if you weren’t squishy?”

“I told you-”

Twilight rolled her eyes. “What if you were as hard as stone yourself?”

“Aside from being popular? Well, I’d probably fare a lot better against the wall.”

Twilight nodded. “That’s right. So if you were flying towards a wall and wanted to go through it, what shape would you make your body?”

“As small as possible, I guess. Just like going through air. Less drag.”

“Right again. If you concentrate all your force on a smaller area, you exert a higher pressure on the target than if you splatted. The higher pressure gives you a better chance of punching through.” Twilight drew a diagram of a smaller pegasus knocking a hole in a wall.

“Wait, you’re saying if I get smaller that will make me penetrate things deeper?” Rainbow frowned. “Are you sure?”

“There’s another way to get more penetration,” said Twilight. “If you’re flying towards something, how do you generate more force?”

“Go faster.”

“Right. Wow, Rainbow, the one time when that actually turns out to be the answer.”

“Hey, going faster is the answer a lot of times!”

“Except for the fact that you’ll still die in a sudden, embarrassing splat at the end.”

“Are you teaching me how to penetrate things or not?”

“Right, right, we were just talking hypothetically. So back to the lesson, once you’ve got a sufficient pressure to plastically deform the object you’re striking-”

“Wait, what?”

“To bend it so much it can’t bounce back to its original shape. Stone, for example, isn’t very flexible. It takes a lot of force to deform it, but once you do it’s broken.”

“But wouldn’t something softer be easier to penetrate?”

Twilight nodded and smiled slyly. “Are you saying the stone walls are too hard for you?”

“No! I’m just saying I could try to penetrate other things. I don’t always have to go around sticking myself into stone. Like, what if I tried jousting against other ponies? Or even swordfighting?”

“Sticking a lance in somepony, huh? Well, that sounds violent, but it does present a much different challenge.” Twilight began to rapidly scribble notes on the board. “A long, solid pole like a lance can go quite deep, the cone shape gradually stretching the hole apart. This is due to the stresses spreading radially and causing material failure around the penetration point.”

Rainbow made a face. “It isn’t fun when you say it like that.”

“Of course not. Would you want to be the one getting penetrated?”

Rainbow laughed. “Would you be doing it?”

“No, no, I stick to theoretical work. I like to just run experiments with myself.” Changing the subject, Twilight said, “So how much penetration do you think is enough?”

“As much as I can, of course. Why is that even a question?”

“Well, if you penetrate too much you could get stuck. You could also go through and hit something on the other side.”

“Like what?”

“Like the other wall of the castle. You’d go through the first and hit the second.”

“So on top of getting hard and going really fast, I need to go for double penetration.”

“The first impact would probably make that impossible. Not to mention it would probably be even more difficult due to being deformed by the first impact. A blunted object, or one breaking apart, would distribute its force over a wider area and therefore reduce the pressure it could apply. An object would have to be traveling at a completely ludicrous speed to go through a castle wall and still carry enough force to go through the second.”

“Twilight, you’re the most powerful magic pony in Equestria and I’m the fastest pony. I think realism went out of the conversation a long time ago.”

“Fair point. Okay, so once you’ve double-penetrated your target like a spit roast, then what? Assuming you had actually generated enough force to pull it off, then coming out the backside you’re still probably moving at a pretty good clip. I know we’ve been talking about destroying things here, but what if there’s something sensitive you don’t want to hit? What about an innocent bystander?”

Rainbow shrugged. “Then I’ll penetrate them too.”

“Rainbow! You can’t just go around sticking yourself in any pony that gets in your way!”

“Okay, jeeze. I’ll ask them first.” Rainbow paused. “But in the interest of the conversation, what would happen?”

“Due to elastic properties, a high velocity impact with a soft target would create a hole artificially larger than the object penetrating it, like a bullet wound.”

“So you’re saying if I stuck myself in somepony hard enough, the hole would get bigger and make it easier?”

Twilight gave her a look. “I’m trying to teach you the physics of bodies in motion here. I’m not sure you’re taking the right lesson. Why would you want the hole to be larger?”

“Because I’m ripping it open. I thought we’d established that I really like penetrating things.”

“How do you know? You’ve never actually penetrated my castle walls.”

Rainbow grinned. “But you just taught me the best way. It’s like you’re practically giving me permission.”

“Well, on the off chance that you actually do manage to go deep and come out the other side, look out for hapless bystanders. Surprise penetration from behind is the worst kind.”

“What do you know about that?”

“I’ve always studied the subject extensively. Princess Celestia taught me about that when I was back in Canterlot.”

“Isn’t Princess Celestia a little busy with running Equestria to think about penetration?”

“You might be surprised. She’s drilled quite a few things with that long horn of hers, a lot of them probably by accident.”

Rainbow snickered. “That must be embarrassing.”

“I can’t imagine. That’s probably why all the ceilings at Canterlot Castle are so high.”

“No, I’m talking about getting penetrated by an alicorn.”

Twilight glared at her. “Careful. I might decide theoretical work doesn't satisfy me anymore.”