For the entirety of last year, everyone in the school had been tormented terribly by the Portrait Curse. Even Flitwick had been misled by the sense of space conveyed by the portraits. Though he had not gone so far as Bill, who believed there was an advanced Extension Charm behind the painting, his thoughts had still extended into other fields—like how the Big Bang expanded from a single point into the universe as it existed today.
There was a Muggle film called Batman. Whenever Gotham faced a crisis, the police would run to the rooftops and light a lamp. It would leave Batman's symbol in Gotham's perpetually cloud-covered sky. Upon seeing the symbol, Batman would drive his sports car out of his cave to save the city whose order had been disrupted by criminals.
There was another superhero, Superman, who could fly. Actually, Superman could not fly; gravity on his home planet was much stronger than on Earth, so a light leap made him seem as though he could fly.
In short, both Batman and Superman had symbols on their chests. Suppose someone made a toy that looked like Batman from one angle and Superman from another, then placed it before a mirror. When the toy showed Batman's symbol from outside, the mirror would show Superman's symbol; when it showed Superman's symbol from outside, the mirror would show Batman's.
A mirror actually always reflected the back of an object. This might confuse people: they were clearly facing the mirror head-on, so how could it be reflecting their backs? When the observer turned around and faced away from the mirror, they would be back-to-back with it.
An object showed different faces when viewed from different angles.
Batman, who came from a super-rich family, had no superpowers and had lost his parents.