Steady flow past a round object does not leave a steady wake. It sheds vortices from alternate sides at a frequency set by one number — St near 0.21 across a wide range of flows — and they line up in two staggered rows. Von Kármán proved only one spacing is stable, rows 0.281 of a wavelength apart; the street on screen settles at 0.280. A temperature probe in a pasteurizer line is this cylinder. If its shedding frequency meets the probe's own natural frequency it fatigues and snaps — which is why thermowells are sized to ASME PTC 19.3 TW.