Have you ever asked God for help in a time of trouble?  Here, the Israelites leaving Egypt are in a bad situation.  They are surrounded by poisonous snakes, that they believe are a consequence of their grumbling against God.  God promises that if they look on a bronze serpent Moses created, that they will live.  Jesus references this story, comparing himself to this bronze snake.  As Jesus is lifted up on the cross, people can look to him and be saved from sin. 
   Old Testament reading:  Numbers 21:4-9
   Gospel reading:  John 3:14-21