During a televised segment of the Rogers Commission hearings (on the 11th of February, 1986), Dr. Richard Feynman pointed-out a likely reason why Challenger exploded.
Conducting a simple experiment—shown in close-up detail, in this video, via an insert—Dr. Feynman revealed that the O-rings were made of a material which was not resilient in cold weather.
This is the actual footage—coupled with an explanatory experiment clip—when Dr. Feynman's questioning of Larry Mulloy helped the world to understand what had doomed the STS 51-L mission.
Clip from the Rogers Commission hearing, featuring the questions of Dr. Richard Feynman.