Here is a mind-bender for you.
The Gaia Ambigram shown here is a nice short symmetric ambigram that is a good example of the letters in one orientation flipping over to become different letters in another orientation.
As you can see you can read this as 'Gaia' when the design is upright and also as 'Gaia' when the design is inverted (or is it the other way around?).
For a mental work-out, try to read it upside-down without rotating it (you can mouse over it at any time to make it spin).
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