How to explain to our young quite how momentous the image of the Queen shaking hands with the former IRA commander, Martin McGuinness, truly is?

How does one tell them that to grow up in the 70s and 80s was to believe that "the Troubles" in Northern Ireland were a problem as apparently intractable as Israel and Palestine? That there was violence, constantly? Violence that spilled on to the mainland through horrific bomb attacks on Harrods, Westminster and more.

Does telling them that 3,500 people were killed in NI between 1969 and 1998's Good Friday Agreement do it? Or that 20,000-plus were injured? I don't know.

One sad truth of "the Troubles" was that news of atrocities could become "wallpaper" in the way of many long-running conflicts: from Vietnam to Afghanistan.

Some atrocities shattered that complacency, be it Bloody Sunday or the murder of the Queen's close second cousin (and Prince Philip's uncle) Lord Mountbatten.

Martin McGuinness and Gerry Adams were regarded as key IRA leaders (though Adams disputes this). Some cannot forgive these men, who now lead Sinn Fein, for their pasts — no matter what they went on to contribute to peace. Nor can some Republicans have any truck with the Queen. Britons mostly live free of this terror today because leaders on both sides were brave enough to make personal, risky stands, the Queen and Mr McGuinness among them.

It is indeed a handshake I never thought I would see in my lifetime. Although it will mean much more to some, this is an historic event that should mean a great deal to us all, young as well as old.