John Lennon's death 30 years ago was one of those shocking, poignant "where were you when" moments that fashion collective memories out of historic events.

Many are sudden deaths - that of JFK being the most notable. Outbreaks and conclusions of war, scandalous resignations of heads of state, sporting miracles, and audacious crimes can all become a "where were you when".

But why do we remember where we were when we heard Princess Diana had died, or Margaret Thatcher resigned? Here some of those who have been influenced by these extraordinary occurrences explain why some stick so vividly in the mind and live on in our imagination....