Two identical bolts are placed together so that their helical grooves intermesh (see illustration). If you move the bolts around each other as you would twiddle your thumbs, holding each bolt firmly by the head so that it does not rotate, will the heads move inward, move outward or remain the same distance from each other? The problem should, of course, be solved without resorting to an actual test.
Amanda Montañez
The heads of the twiddled bolts move neither inward nor outward. The situation is comparable to that of a person walking up an escalator at the same rate that it is moving down.
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A version of this puzzle originally appeared in the August 1958 issue of Scientific American.