From the Washington Post this morning, an overwrought piece about the miracle of sandbags. Wade through the overly ambitious prose, and you’ll find some interesting information about sand, sandbags, and the battle against rising water.

Sand is defined by soil scientists as disintegrated rock particles between 0.05 and 2 millimeters in diameter. Good sandbag sand is at the finer end of that scale. It packs well. Water does not flow through it easily. When the water-resistant but not waterproof bags let enough water in to soak the sand, they become almost as hard as rocks. If sandbags fail you, generally it is only because you neglected to pile them high enough.

But sandbags can do more than keep you dry. Enough sandbags will stop a bullet. During the Vietnam War, soldiers built great cathedrals of sandbags, with interior chambers and stereo systems.