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From films and debris, it appeared that the solid rocket motors had failed first, sending a blowtorch of hot gas into the external tank, which then exploded. The solid rocket motors were built of a stack of components containing the solid fuel, which were then joined to make the whole rocket motor; it appeared, in fact, that one of the joints had failed.

One proposed explanation was that the cold has made the O-ring seals at the joints stiff. During a public, televised hearing, management people from the solid rocket manufacturers discounted this idea. Feynmann, who was one of the members of the all-star panel doing the investigation, quietly got a salt shaker and a glass of ice. They had a sample of the O-ring material that had been provided as a prop for the hearing. Feynmann put the salt into the ice, making a concentrated salt solution with a temperature much lower than the normal freezing point of water. Feynmann, without making a fuss about it, dropped his sample of O-ring in the water and let it chill.

Here’s the strong-inference part of this. The Thiokol managers’ hypothesis was that the O-ring material remained “sufficiently” flexible at the temperature it would have reached on that unusually cold Florida morning. Feynmann’s experiment simply said “okay, so let’s get a piece of this stuff cold and see what happens.”

The answer, which Feynmann proceeded to demonstrate in a nationally televised hearing, was that the stuff got to be very brittle. Feynmann took what had been a soft, rubbery material at room temperature, and it broke like glass.

So much for the managers’ hypothesis.