And God is pure; there is no sin in Him. He is Holy.
But human beings are full of shameful sinfulness. When God first made the world, there was no sin. Adam and Eve, the first man and woman, were created pure and clean—sinless. God walked with Adam in the Garden of Eden. Adam could look at God's face and enjoy being with Him everyday.
But then, Adam and Eve did a terrible thing; they disobeyed God. So did their children after they were born. You and I are Adam and Eve's great, great, great, great, great, great, great… grandchildren, and we are full of sin, too.
Sin is the cause of all the bad troubles and suffering in our world.
Sin is a terrible, terrible thing. Because of our sins we are separated from God. He is clean and pure and Holy; human beings are polluted and stained with sin.