One historical concept i find fascinating is that of "living memory", by this i mean historical events that were witnessed (either first or second hand) by people still living. While very recent events have millions of living witnesses all historical events eventually pass from living memory as people who were alive during these events die off.
World War 1 for example will soon pass from living memory. It is not thought any soldiers who fought in the conflict are still alive since the passing of veterans like Harry Patch, there will still be some civilians who were alive during the war who will have memories of the times (even if they were no where near any actual fighting). Even the youngest of these (say someone who was 5 or 6 in 1918) will be over 100 nowadays.
Second-hand memory can persist for longer. By this i mean memories being passed on by first-hand witnesses to their descendants (orally face to face). In some ways this retains a living link to events that passed from living memory many years before. Interestingly the American Civil War even though it ended in 1865 is still in second-hand memory after news of a couple of children of veterans still being alive and still receiving a war pension.
The last veteran of the war died in 1956 though many veterans did marry much younger women and the last widow did not die until 2003. It is likely these surviving children were the result of older veterans marrying young women (which was not uncommon in the early 20th century). How much these 2 survivors can remember of what their fathers may have told them about the war is unknown, it is reported that both are of poor health and very old, but they remain a living link to a war that passed from living memory half a century ago.