As in the strings in string theory? Unknown. Possibly just highly condensed "energy".
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If you're talking about String Theory (M-Theory), they aren't really made of anything but energy.
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stringy stuff?
tennis racket strings were traditionally made of "cat gut" which was animal sinews (tendons)
tennis racket strings were traditionally made of "cat gut" which was animal sinews (tendons)
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Varies:
cotton
help
nylon
polypropylene
cotton
help
nylon
polypropylene
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energy