A spoonful swapped between wine and water — which glass 'wins'?
Two glasses hold equal amounts of wine and water. Take a spoonful of wine and stir it into the water glass. Then take a spoonful of that new mixture and stir it back into the wine glass. Without knowing the spoon's size, or how thoroughly it was stirred either time: is there now more water sitting in the wine glass, or more wine sitting in the water glass?
Reveal the answer
They're exactly equal, regardless of the spoon's size or how well anything got mixed. Both glasses end up holding their original volume again, so whatever wine is missing from the wine glass has to be somewhere — and the only place it can be is the water glass, displacing precisely as much water as crossed over the other way. Martin Gardner popularized this in his Scientific American column (1957), collected in Hexaflexagons and Other Mathematical Diversions (1959).
— Martin Gardner (popularizer), Hexaflexagons and Other Mathematical Diversions — Scientific American column, September 1957; collected 1959
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