I've always been disappointed that number theory, set theory, etc aren't introduced in middle school or high school. It makes sense, since those are a lot less

I've always been disappointed that number theory, set theory, etc aren't introduced in middle school or high school. It makes sense, since those are a lot less useful than the subjects that are taught, but something like number theory is incredibly approachable to a middle school student. And it can show students that math can be a lot less about memorization and a lot more about creative thinking w.r.t. proofs.