Prime Minister speaks to Newsround about World War One
5 August 2014 Last updated at 08:30 BST
Prime Minister David Cameron has told Newsround how difficult it was to explain World War One to his own children.
Lights were turned off across the UK last night, to mark 100 years since Britain became involved in the conflict, by declaring war on Germany.
And yesterday, at a special ceremony in Belgium, Ricky spoke to Mr Cameron about the lessons we should learn from the Great War.