] INDIANAPOLIS, Indiana (AP) -- Eat your way to the bottom
] of almost any bag of popcorn and there they are: the
] rock-hard, jaw-rattling unpopped kernels known as old
] maids.
]
] It's long been known that popcorn kernels must have a
] precise moisture level in their starchy center -- about
] 15 percent -- to explode. But Purdue University
] researchers found the key to a kernel's explosive success
] lies in the composition of its hull.
]
] Unpopped kernels, it turns out, have leaky hulls that
] prevent the moisture pressure buildup needed for them to
] pop and lack the optimal hull structure that allows most
] kernels to explode.

Finally! The mystery is solved! I, for one, can die happy.

CNN.com - Scientists solve unpopped popcorn - Apr 21, 2005