Imagine you're trying to impress your friend Olive with how healthy you are. "Oh, yeah, spinach, I love that stuff," you say. "And chocolate? Pfft, don't like it at all."
The next day you're at a store (in the strictest versions of the theory it needs to be a store that only sells spinach and chocolate - you know, just your average Spinach 'n' Chocolate Emporium). When you get home with your bag of groceries, Olive notices that it's chock-full of chocolate. "A-HA!" she shouts excitedly. "No matter what you claim to like, you have a revealed preference for chocolate over spinach." You hang your head in shame... but at least you have some comfort chocolate to nibble on!
Further reading: Wikipedia
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Uri Bram writes popular non-fiction books with a conceptual approach to mathematical, scientific and analytical thinking. He is the author of Thinking Statistically and Write Harder.
Cover image: Christmess, by makelessnoise