What phenomenon did Feynman demonstrate at the Challenger disaster inquiry using a glass of ice water?

At the Challenger inquiry, Richard Feynman demonstrated that the shuttle’s O-rings could become brittle and lose flexibility in cold temperatures. The simple experiment helped explain how cold weather contributed to the disaster.

On January 28, 1986, Space Shuttle Challenger broke apart shortly after launch, killing all seven crew members. Investigators focused on the solid rocket boosters, which used rubber O-rings to seal joints between sections. On the unusually cold launch morning, the seals did not respond as designed, allowing hot gases to escape and damage the vehicle.

During a televised hearing of the Rogers Commission, Feynman placed an O-ring in ice water and then showed that it had lost much of its resilience. The demonstration made an abstract engineering failure visible to the public, though the complete accident involved design, testing, management, and communication failures as well. The issue was not that O-rings conducted electricity, dissolved in water, or simply expanded in heat; their critical problem was reduced elasticity and delayed recovery in the cold.