It’s said that British Astronomer Royal G.B. Airy once discovered an empty box at the Greenwich Observatory in London.

He wrote EMPTY BOX on a piece of paper and put it inside.

“Attached to the outside, such a label is true,” write Gary Hayden and Michael Picard in This Book Does Not Exist. “Placed inside the box, it makes itself false. Alternatively, suppose the label says: ‘The box this label is inside is empty.’ Outside of any box, the subject of this sentence fails to refer — there is no box inside which the label is located. However, once inside an otherwise empty box, the sentence becomes false.”