These are some popluar badges
This badge first popped up on Etsy and has since emerged on NotCot and Boing Boing. Not surprisingly it has sold out.
If only I had decided to use my philosophy degree more profitably by making badges. For more background on the liar paradox have a look on Wikipedia.
It shows that creative ideas can take any form and often can make money in their own right. Self funding creative ideas. Now there's an idea.