Twiddle Two Bolts Together — Which Way Do the Heads Move?
Two identical bolts sit head to head with their threads meshed together like gears. Hold each bolt firmly so it can't spin in place, then twirl them around each other the way you'd twiddle your thumbs. As the bolts orbit one another, do their heads drift closer together, farther apart, or stay exactly the same distance apart?
Reveal the answer
They stay exactly the same distance apart. Neither bolt actually rotates as it orbits the other (a mark on each head keeps pointing the same way), so one bolt effectively screws itself in while the other unscrews itself, at perfectly matching rates. Known as 'Twiddled Bolts,' from Martin Gardner's August 1958 Scientific American column, later collected as Puzzle 18 in My Best Mathematical and Logic Puzzles.
— Martin Gardner, My Best Mathematical and Logic Puzzles — Scientific American, August 1958 (Mathematical Games column); collected as Puzzle 18 in My Best Mathematical and Logic Puzzles (Dover, 1994)
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