The earthquake hits while Mrs. Gretten and Mr. Hill are engaged in a quickie in the chemistry supply closet. It’s just a normal California shiver and only a couple of things fall off the shelves behind them.

These comprise two packets of silver nitrate, a small cascade of instructional DVDs and a beaker of sulfuric acid. It’s a Pyrex beaker, and sealed; it bounces off that indestructible black resin countertop once without breaking.

Then Gretten and Hill scream, because the beaker bursts in midbounce. It’s been struck by something very small, moving very fast.

But not as fast as Chicago’s shutter.