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Start swiping — it's freeLeibniz argued that no two distinct objects can be exactly alike in every respect: if you could list every single property of X and every single property of Y and the lists matched perfectly, X and Y wouldn't be two things at all, just one thing described twice. It sounds obviously true until you try to apply it to physics, where quantum particles of the same type seem to share every measurable property yet still count as separate particles — a case philosophers still argue over today.
— Gottfried Wilhelm Leibniz, Discourse on Metaphysics — 1686
Take a long strip of paper, fold it into a flat hexagon, and glue the ends. It looks like it has just two faces. But pinch three alternating corners together and fold it open from the centre, and a third face appears, one that wasn't visible a moment ago. Keep flexing it the right way and you can cycle through even more hidden faces. How many faces can a hexagon built this way actually have?
A basic "trihexaflexagon" has three faces; more elaborate versions built from longer strips can hide six, nine or more. The shapes were discovered in 1939 by British student Arthur H. Stone, who was trimming American paper to fit English binders and started folding the offcuts. He and fellow Princeton students Bryant Tuckerman, Richard Feynman and John Tukey formed an informal "Flexagon Committee" to study them. Martin Gardner's very first Mathematical Games column for Scientific American, December 1956, was about hexaflexagons — its popularity launched his 25-year run writing the column.
— Martin Gardner, Flexagons — Scientific American, December 1956
Ivan Pavlov noticed his lab dogs drooled at the mere footsteps of the assistant who fed them, not just at the food itself. By repeatedly pairing a neutral sound with feeding, he showed dogs learn to salivate at the sound alone, the founding demonstration of classical conditioning, still the backbone of how psychology explains learned associations.
— Ivan Pavlov, Conditioned Reflexes — Lectures 1897-1903; published 1927
In 1665, Robert Hooke examined a thin sliver of cork under a microscope he built himself and saw tiny walled compartments, which reminded him of the small rooms monks lived in. He named them 'cells' in his book Micrographia, the term biology still uses for the basic unit of life. He was actually looking at dead plant cell walls, not living cells, but the name stuck for good.
Ushuaia sits on the Argentine side of Tierra del Fuego, at the southern tip of South America, making it the southernmost city of its size in the world. Most Antarctic cruises and research expeditions depart from its harbour, since it's the closest major port to the frozen continent by a wide margin. Its nickname, 'el fin del mundo,' the end of the world, is stamped on postcards, passports and tourist stalls across the town.
— Wikipedia contributors, Ushuaia — Standard geographic accounts