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The world-can be a crazy place
You forget the things you want to say
And let the moment slip away
The author is trying to say that the world is such a huge space that can change one's train of thought that we can forget the things we really wanna say and next thing you know, that important moment where you wanted to say something is gone and can't come back anymore. The moment slipped away, the moment you wished to say something is now gone and the world being a crazy place had an impact on that.