One time, some tax collectors and other outcasts sat with Jesus while he told them stories.
Some religious leaders complained about this and said, This man Jesus talks to tax collectors and other outcasts.  He even eats with them!

Jesus knew what they said so he told them this story: 
If a man has a hundred sheep and one of them gets lost, what does he do?  He leaves the rest of his sheep and goes out to look for the lost sheep.

When he finds it, he is happy.  He picks up the sheep and carries it back home. He is so happy that he calls all his friends to come help him celebrate.

It is just like this in heaven.  Everyone is happy and celebrates when even one sinner repents and turns toward God.
