Because spheres can be packed snugly together in certain configurations, one would expect the atoms of inert gases, such as helium, to cluster together in similar tightly packed groups. The numbers of spheres or atoms in these geometrically and energetically favored clusters are termed "magic." One would expect, for example, the number 13 to be magic because this would be the number of spheres fitting tightly into a 20-sided solid. Sure enough, when clusters of helium atoms are "weighed" with a mass spectrometer, the number 13 is strongly favored; so are 19, 25, 55, 71, 87, and 147.

Some of these experimentally derived magic numbers can be predicted theoretically, but others were surprises. Some theoretical magic numbers did not turn out to be magic in reality; notably 7, 33, and 43. Clearly nature has other criteria for "magickness" than the physicists.

(Anonymous; "Magic Numbers Do Hold Atoms Together," New Scientist, 92:598, 1981.)