Wittgenstein's rod
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Wittgenstein's Rod is a thought experiment attributed to Ludwig Wittgenstein.
A rod passes through a sleeve, in which it can smoothly slide. The sleeve is fixed to a wall, forming a pivot. One end of the rod is moved in a circle. What shape does the other end of the rod describe?
The seemingly obvious answer is a circle, but in fact it describes an inverted cardioid.
