Brushes don't have any interior. When you first time draw a brush, it has 6 surfaces, and that's it. When you cut a brush, depending on your preferences the editor might prefer to caulk the created two new surfaces or cover them with the same texture you have on the other surfaces of the brush (or some other logic like that). And that "paper-thin" texture (surface) is everything the game needs.
Otherwise brushes are hollow like the talk of politicians: From the outside they can look nice and solid, but if you could look inside them, there's nothing there.
Edit: And if it's a fruitcake, there's not necessarily any nuts in it...
Frozen by ICARUS