I just thought of this one, maybe it isn’t the most glamorous but it works.
In a magic show, a magician tells the audience about a special mirror they have, a mirror that isn’t a mirror, that doesn’t reflect anything. Crazy, right?
The audience thought so too, and demanded to be shown this strange mirror.
The magician takes the mirror out, a simple, unadorned, gleaming mirror, it looks just like any old mirror.
The audience is incredibly doubtful, as the mirror very clearly reflects everything. The mirror is clean, and they even show the back of it, which, is also normal.
No tricks, the magician insists. They step away, showing nothing in either hand, sleeve or in any other compartment on them. Then the magician does something, which indeed makes the mirror reflect nothing.
What did they do?