We think this idea dates from somewhere in the 1980s. A very basic eathenware mug, it is just the decoration that shows a nice trick.
Inside the mug, around the top edge, are the words 'Mirrors Reverse Images'. Below the words there is a mirror surface - we understand that this is a platinum lustre that has been applied to the inside of the mug.
Opposite the mirror, inside the mug, there are some words - 'But not words in mugs'. When these words are viewed in the mirror, they are still readable - they are not reversed. How can this be?
Well, while it is true that normally a mirror will reverse an image, and will also reverse text, this does not apply to a curved mirror, which effectively re-reverses the text, making it the right way round again!
Reflections in spoons can show the same effect. The main challenge here is a technical one - applying the platinum lustre. Apparently when you fire the mug you tend to get platinum vapour spreading, so really it is best to have a separate firing, otherwise other items in the kiln are all going to get a slight lustre!