An Important Distinction
Something very strange often happens when people come to the scripture. People who are in every other circumstance able to distinguish different parts of speech, such as sarcasm, metaphors, hyperbole, and others, suddenly forget that these exist when it comes to scripture. There are a few likely sources for this. One is simply that we treat the Bible differently than other books. This is not in itself a bad thing. The Bible is not like other books, it is God's book, given to His people so they might know Him and know what He has commanded them to do. The issue arises when we fail to see that the Bible uses the same language patterns as other books. There is poetry, prose, history, parables, metaphors, and more. Another issue, which in some ways is an extension of the first, is that we think of historical people as very flat. We see them as a list of details on a page and often forget that these people had full lives, filled with joys and sorrows, hopes and dreams. Because of our sin and selfishness, we find it hard to think of other people having the same thoughts and feelings as we do. Whatever the reason, we need to understand that the Bible uses literary devices for our benefit and to teach us more fully who God is and what He has commanded.
An Aristotelian Error
"Are then the bread and wine changed into the real body and blood of Christ?" (Q.78)
To some, today's catechism question may seem silly, and to others, it may not. But for the writers of the catechism, this was a serious issue that could cost them their lives. The Heidelberg Catechism was originally written in 1563 during the height of the Protestant Reformation in Germany. The Roman Catholic church taught that the elements of the Lord's Supper, the bread and the wine, truly became the physical body and blood of Christ. The reason why they taught this is a lengthy topic, but the short version is that they based their understanding not on the teaching of scripture, but on the teachings of a Greek philosopher named Aristotle. Aristotle taught that everything essentially was made up of two parts, substance, and appearance. So the Roman Church taught that when the priest said the magic words, "Hoc est corpus meum," Latin for "this is my body" (as a side note this is where we get the idea of the magic words being hocus pocus) the bread and wine were miraculously transformed in their substance to the body and blood of Christ. The Reformers rejected this error. So the catechism answers;
"No. Just as the water of baptism is not changed into the blood of Christ and is not the washing away of sins itself but is simply God's sign and pledge, so also the bread in the Lord's supper does not become the body of Christ itself, although it is called Christ's body in keeping with the nature and usage of sacraments."
What Does Happen?
While there is still some disagreement on this question between Protestants, particularly between Lutherans and most other Protestants. I believe the testimony of scripture is clear on the subject and is witnessed in the catechism's answers. The bread and the wine are simply that, bread and wine. There is nothing particularly special in the elements themselves, but are important for what they represent when they are used as part of the Lord's Supper. When we get baptized the water does not change or do anything different than it does when we take a bath, but what is being represented and witnessed is the difference. It takes on a greater meaning because of what it points to. It shows us a visible representation of our union with Christ. When we take the Lord's Supper together as the body of Christ, we celebrate the death of Christ until He returns. He does not die again, and He does not become physically present, but when the church gathers together, He is always with us.
When we do not understand that the Bible uses metaphors to help us understand things, we can miss the point and put the emphasis in the wrong place. We can end up making more of the symbol itself than what the symbol points to. The Lord's Supper is a celebration and a reminder of who Christ is and what he has done. We are called to taste and see that the Lord is good and that his steadfast love endures forever. But the symbol is not the Lord himself. The Lord is truly present when we celebrate his ordinances, but we should not take that to mean we are actually eating the body and blood of Christ. The Lord has been good to us, giving us visible reminders of his promise in baptism and the Lord's Supper. We should join regularly with the Church, the body of Christ, to celebrate baptism and communion. These visible reminders strengthen our faith and encourage us as we look forward to the day we will no longer need such reminders since we will be with the Lord of Glory himself.
Soli Deo Gloria
 Eph. 5:26; Tit. 3:5.  Matt. 26:26-29.  I Cor. 10:16, 17; 11:26-28.  Gen. 17:10, 11; Ex. 12:11, 13; I Cor. 10:3, 4; I Pet. 3:21.