I think it's now a dumb running joke to test the limits of my brother's definition of a sandwich.

A sandwich is a filling surrounded by bread. Bread being baked from dough made from flour, water and yeast.
So in his eyes, a BLT is a sandwich, a Subway sub is a sandwich, a hotdog is a sandwich, all pretty normal so far.

When I ask about ice-cream sandwiches and wraps, he specifically declines because in both cases, neither is made with "bread".

So I press on, and ask.
Are two pizza slices held facing together a sandwich?
He pauses.
He agrees but then turns around to say that the toppings on a pizza become part of the bread when cooked and thus, is no longer a separate "filling".

Disregarding that line of logic, I try to argue that bread shouldn't be defined by "yeast" since there are breads baked without it. He agrees to change the definition to "leavening agent". This is his mistake.

So this thread is just the set-up to this joke in my head.
Plato: "A sandwich is a filling surrounded by bread. Bread is baked from flour, water and yeast."
Diogenes, holding a slice of cheese between two brownies:
"BEHOLD, A SANDWICH."

As a result of this conversation, my leavening agents privilege has been cut, and bread is no longer a term of discussion with my brother anymore.