Ok kiddo, let me explain the Downing Street Memo to you. You know how sometimes grown-ups have meetings to talk about important things? Well, a long time ago, some important grown-ups in Britain had a meeting at a place called Downing Street. They talked about something called war.
Now, you know that war is when people fight each other and it's not good because people get hurt and things get destroyed. So, these grown-ups were talking about a war that was happening far away from Britain. They were saying that someone called the United States wanted to start this war and they were asking Britain to help them.
Here's where the Downing Street Memo comes in. One of the grown-ups at the meeting wrote a memo - which is like a special note to remember what they talked about. In the memo, the grown-up wrote that even though the United States said they had good reasons to start the war, they actually didn't have enough proof that they needed to start the war.
Basically, the memo was saying that the United States was making up reasons to go to war and Britain was being asked to help. This was a big deal because lots of people didn't want the war to happen, but the United States went ahead and started the war anyway.
The Downing Street Memo became important because it showed that some grown-ups knew that the reasons for the war might not be true. And lots of people thought that the war was wrong and shouldn't have happened.
So, that's the Downing Street Memo. It was a memo written by an important grown-up in Britain who said that the United States was making up reasons to start a war.