Why did Jesus use bread to turn into His body? In the Catholic church you have communion, in which you receive the body and blood of Christ in the form of bread and wine.  Why do bread and wine become the body and blood of Christ, rather than other elements?
One aspect I don't see addressed in other posts is the significance of bread and wine. Yes, it's because that's what Jesus said to do, but he did not merely pick to random substances to represent himself. 

Although I would argue that John 6 does not have sacramental overtones in mind per se, we do see Jesus using bread to represent himself. A couple of observations from that chapter:


"Bread" is used as shorthand for the manna given in the wilderness (see v31), as pointed out by the people. Jesus uses this to show them that bread in general, and specifically the True Bread, is given by God
Bread was the main sustenance of the day. Food in general is that without which you die. Yet the food itself is a living thing, so either it dies, or you do. In other words, bread in general and the True Bread must give up its life for you to have life
Jesus uses bread to contrast something that must be done repeatedly (eating daily), with partaking in him, which is done once (v35). He pushes the metaphor then away from simply eating to security as children of God. Bread is taken repeatedly and still does not prevent death. Likewise, the Jews sacrifices were performed repeatedly, but do not ultimately remove sin. But with Jesus, his one act is sufficient to secure one's place in heaven with him by faith. (see, e.g., v39)