Sin is choosing to do what we know is wrong. When we sin, we say no to God's love, and we hurt our friendship with God and with other people.
Some sins are very serious. A mortal sin is a choice to do something seriously wrong, knowing it is wrong and freely choosing it anyway. It breaks our friendship with God. Less serious sins, called venial sins, weaken that friendship.
Accidents and mistakes are not sins. Sin is always a choice.
Jesus told a story about a father with two sons. The younger son asked for his share of the family money, left home and wasted it all. Soon he was so hungry he was feeding pigs and wishing he could eat their food!
Then he remembered his father's house and decided to go home and say sorry. While he was still far away, his father saw him, ran to him, hugged him and threw a big feast. “My son was lost and has been found!”
Jesus told this story to show us God's mercy. God is like that father, always watching and waiting to welcome us home.