I was at work. We had an engineer who used to come to fix and service some of our lab equipment who was notorious for telling stupid and sometimes really sick jokes. He'd just heard the news on his car radio as he was arriving at our place, so his first words were 'Have you heard a planes crashed into one of the twin towers?' - and as usual, we were waiting for the punchline, while he was pleading with us that this time he was actually telling the truth.
We realised he was telling the truth when his wife called him on his mobile to tell him about the second one. There was a sense of disbelief/unreality about it which gradually turned to shock.
never forget