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Materials that shouldn't exist under normal conditions but somehow do—room-temperature superconductors, stable metallic hydrogen, transparent aluminum, and other substances that would revolutionize everything if they could actually be made. The phrase is scientific shorthand for "things we've theoretically predicted but cannot practically produce," or more cynically, "grant proposals that will be funded for another decade." Exotic materials at ambient temperature and pressure would enable lossless power transmission, hovering vehicles, unbreakable everything, and a permanent place in the Nobel Prize committee's heart. Their absence from your daily life is a reminder that nature doesn't give up its secrets easily, and that "theoretically possible" is not the same as "actually feasible."
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