They knew God said not to eat the fruit from the tree of the Knowledge of Good and Evil. But they did it anyway. They turned away from the way God wanted them to go. They turned away from God.
Adam and Eve brought sin into God's perfect world.
Sin is knowing what God wants, but doing what you want instead. Adam and Eve disobeyed God, and at that moment, everything changed.
Even though Adam tried to blame Eve for eating the fruit, and Eve tried to blame the serpent, God knew the truth. They all had disobeyed him.
So first, God said to the serpent,
"Because you lied to my children and tricked them into disobeying me, you will crawl on your belly from this day on, and you will eat dust as long as you live."
And then, because God knows what is going to happen today and every day after that, he said to the serpent (remember, the serpent was really Satan), "You will hate Eve and her children, and all the people who are ever born after them. You will be their enemy. And you will do everything you can to hurt them. You will do everything you can to keep them from knowing how much I love them. But one day a child will be born. You will bite his heel, but he will crush your head."
The child that God was talking about is Jesus.
Way back at the beginning of time, God already knew that one day Satan would hurt Jesus - on the day that Jesus died on the cross. But just as sure as that, God knows of another day that is still coming - the day when Jesus will get rid of Satan forever.
Now God turned to Adam and Eve.
God said to Eve, "Because you sinned and disobeyed me, this is what is going to happen. When you have children, it will not be easy. It is going to hurt. "
And he said to Adam, "Because you disobeyed me, you will have to work hard all your life for the food you eat and the things you need. Weeds and prickly things will grow along with your food, and you will sweat to make things grow. All your life you will have trouble."
Their hard life would always remind them what sin can do.
And he said to both of them, "And then one day you will die, just as I said you would. I made your bodies from the dust from the ground, and one day they will become dust in the ground again."
All this happened because Adam and Eve disobeyed God.
It wasn't that God was being mean or unfair. God had made a place where everything worked together perfectly. And it was wonderful. Everything was just the way God wanted it. But when Adam and Eve sinned, they chose to to do something different from what God wanted. Now everything wasn't working together so perfectly. And that messes up everything. That is why sin is so terrible.
So God made clothes out of animal skins for Adam and Eve to wear. And then God sent them out of the beautiful, perfect garden he had made for them. God put an angel with a flaming sword on guard at the graden's gate to protect the perfect place from the terrible things sin can do.
Adam and Eve began to learn that when we choose to do what we want instead of what God wants, we ruin the good things God has made for us.
But even though Adam and Eve had sinned, God still loved them.
The very moment Adam and Eve disobeyed God, sin came between God and his children.
Because we are all God's children, sin has separated us all from God. We can't live apart from God, so now God had to make a way to bring us back to him. And that is what the rest of the Bible is all about!