Columbus' egg; egg of Columbus
Refers to a seemingly brilliant idea that appears simple only after it has been demonstrated. Often used when someone belittles an innovation by saying they could have thought of it themselves.
His invention was called a Columbus' egg.
成功してから「そんなの簡単だ」と言うのはコロンブスの卵のようなものだ。
Saying 'That's easy' after someone succeeds is like the egg of Columbus.