Where were you when the Space Shuttle Challenger exploded?
It was a cold January morning, and we had a snow day. Shuttle launches had been going on for 5 years, and were really routine. I don't even remember if this was on TV anyway.
I was out shoveling the driveway, and when I came back in, Mom tells me, "Something happened to the space shuttle." Now, of course, it's on TV, and so I sit down in front of it, and they replay the tape, the flash of light, the solid rocket boosters shooting off in different directions. You didn't need to be an aeronautical engineer to know that there was little chance the astronauts were alive. I remember saying that to my Mom as we were watching in somber, stunned horror.
Mission control first refered to the explosion as a "major malfunction." In those first few minutes, you knew this was going to be a major historical event.