I remember clearly the first time I stopped to think about how a model ship comes to be trapped inside a bottle. I can't have been more than 8 years old, and I cannot say it crossed my mind to ask why ships end up in bottles in the first place. That's just not the kind of thing that 8-year-olds worry about. So it was a different story many years later when I first came across a three-dimensional image (of a small cherub, as it happens) floating inside a block of glass: while I certainly marvelled at the apparent impossibility of what I was seeing, I also wondered why anyone would have thought it was necessary to use what was clearly an exciting and powerful new technology to create three-dimensional cherubs.

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