If you carefully stack equal-sized marbles in a box, they will fill about 74% of its volume. But if you dump them in, they won't fill up more than about 64%, no matter how much you jiggle the box. Researchers have measured this "random close packing" limit but can't explain it. Now a team reports that the limit occurs when virtually all marbles form into distorted pyramid shapes called quasiregular tetrahedra.
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