"It's really cool! Things can fly if the lift they generate with the movement of their wings is greater than the pull of the earthforce. The lift they generate is a product of surface area and how they move, the easiest way to measure it is actually flapping in a pool and measuring how the water moves."

"Well, I suppose I could go flap my wings in a pool."

"You should! Don't you want to know whether you fly by magic?"

"I can't say I've ever run into a situation where I needed to know! Does magic stop working under some circumstances, must I worry about falling out of the sky?"

"Not that I know of! But I'd want to know anyway!"

"Well, if there's a pool somewhere..."

"There is! We'll also need dyes so we can measure the right things about the water."

"All right."

He goes and fetches those.

She will follow him to the pool there apparently is.

It's through his rooms. In his rooms: a towering spiral staircase that looks out on a skylight that looks out on the night sky, a lot of books, several hundred dead beetles pinned to the walls, and a baby mouse in a waist-high cage, not moving but not dead, just slow. 

He pulls out a sketchpad and sits down next to the water.

"What is that?" she asks of the mouse.

"That? It's a mouse."

"Huh. It's weird looking."

"It's a baby, the adults are too big to keep indoors."

"I think I'd find the adults weird looking too! Why do you have one?"

"There was a nest right near here and I wondered if I could lure it in if I spent most of a day dropping little morsels of food in the right places and I did and it followed them! Now I'm just wondering what it'll do and what its life cycle is like and how long it takes to grow."

"Huh. Anyway, you just want me to get in the pool and flap my wings? Is it all right if my head's above the water?"

"You can be any which way as long as the wings are underwater."

She assesses the depth of the pool, gets in, and flaps.

And he attempts some calculations about the volume of water she's moving.

"Okay then I need to also see it in air to see how much faster you are -"

"I don't have the headroom to take off in here, do you have something I can hold on to?"

"Depends what you need? You could hold onto me?"

"No."

"Okay. We could go some place with a higher ceiling? How high does it need to be?"