The paradox is a wonderful teaching tool. The sleepy student in the back row is surprised and wakes up, and the student with the instantly memorized answer is forced into the analytical mode. The diagram in Fig. 1 has the following paradox: A body sliding freely down a chord from the edge of the circle reaches the lowest point on the circle at the same time as a body released simultaneously from the top. This result was first mentioned in a 1602 letter from Galileo Galilei to Guidobaldo dal Monte.1

1.
Domenico Bertolini Meli, Thinking with Objects (Johns Hopkins University Press, Baltimore, 2006) p. 71.
2.
Richard M. Sutton, Demonstration Experiments in Physics (McGraw-Hill Book Company, London, 1938), pp. 42–43.
3.
Thomas B.
Greenslade
Jr.
, “
All about lissajous figures
,”
Phys. Teach.
31
,
364
370
(Sept.
1993
).
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