One by one, a square is missing a corner, how to add a line to become a 2 triangle

One by one, a square is missing a corner, how to add a line to become a 2 triangle

Don't mind, I'm serious. This is an answer I saw on the Internet by chance. At that time, I thought that this kind of solution was like a Brain Twister, which was not serious. But from another angle, did the questioner want us to break the mindset? In this way, this kind of solution seems to be acceptable