Number 21:4-9

"Then the Lord told him, 'Make a replica of a poisonous snake and attach it to a pole. All who are bitten will live if they simply look at it!' So Moses made a snake out of bronze and attached it to a pole. Then anyone who was bitten by a snake could look at the bronze snake and be healed!" v8-9
People are still complaining! This time God sends poisonous snakes! Some die. This causes the people to realize they have sinned against God and Moses. They ask Moses to pray on their behalf and God instructs him to make a bronze snake and hang it on a pole. When the bronze snake was hung on the pole, the Israelites didn't know the fuller meaning Jesus Christ would bring to this event. Jesus explained that just as the Israelites were healed of thier sickness of sin by looking at the snake on the pole, all believers today can be saved from the sickness of sin by looking to Jesus' death on the cross. It was not the snake that healed the people but their belief that God could heal them. This belief was demonstrated by their obedience to God's instructions. In the same way, we should continue to look to Christ.