Every Catholic making a First Communion learns that the Eucharist is the Body and Blood of Christ, that he is really present (a concept descriptively called the Real Presence), and that the host and wine invisibly change into his literal Body and Blood through something called "transubstantiation."
To the curious and practical minded, that leaves a lot of questions still to be answered. And, never fear, when you have a strange theological question, the Church has always thought of it first. Here are answers to some of those questions you may have been too embarrassed to ask:
1. Is it okay if I take only the host or only the cup? Or do I need both to get the "whole Jesus"?The consecrated host is called the "Body of Christ" and the cup is called the "Blood of Christ," but that refers to their superficial resemblance; the solid bread is analogous to Christ's solid Body and the liquid wine is analogous to Christ's liquid Blood. The fact that one is solid and one is liquid reminds us that we receive both Body and Blood. The answer to the question, however, is that the host and the cup each contain the fullness of Christ's Body, Blood, Soul, and Divinity. Thus, you can receive either one and get the "whole Jesus."
The belief that one must receive both forms to get the "whole Jesus" is a heresy called Utraquism. To paraphrase the Catholic Encyclopedia, we do not believe that Christ limits his real presence, or holds back part of the spiritual nutrition of the Eucharist, based only on the physical appearance of the sacred species. Since transubstantiation means that the bread and wine are completely transformed, it does not make sense that there should be a limitation on what they turn into.
2. Why is the Eucharist not the same as cannibalism?Given that Christ is human and that he is really present in the Eucharist, it does sound a bit like cannibalism. And that concern may be what drove away some of his disciples at the end of the Bread of Life discourse in John 6. But when we consume the Eucharist, we eat Jesus in a sacramental way, not in the mundane way that would be eating a person's physical flesh.
Consider what it means to eat meat. You don't take the animal's substance into you; you don't contain "cow-ness" or "pig-ness" afterward. You contain only the physical molecules that made up its flesh. The same happens in cannibalism. Cannibals do not contain the personhood or "human-ness" of their victims, only their meat.
In the Eucharist, on the other hand, the faithful receive the personhood of Jesus. They contain his soul and divinity in a real, mystical way.
Cannibalism is an offense against the dignity of the human person. Because he offered it to us, because in fact it exists solely for us, eating and drinking the Eucharist can never offend the dignity of Christ.
3. What if I throw up after Communion?Eww. But it's an important question; after all, the Eucharist is often given to sick people, and even people who feel healthy may vomit unexpectedly at times. The main principle is that the real presence of Jesus remains as long as the sacred species retains the physical characteristics of bread and wine. The Eucharist can theoretically be contaminated not only by being vomited from a recipient, but also by becoming dirty, by being poisoned, or by suffering other potential sacrileges. And Christ surely does not expect us to consume a contaminated Eucharist.
In these cases, a priest will, in a dignified and respectful way, remove the appearance of bread or wine; then the contaminated stuff won't be the Eucharist anymore. The content of the cup, the former wine, is diluted with pure water until it no longer resembles wine. A poisoned host is disposed of the same way. A vomited Eucharist, on the other hand, is "gathered up and disposed of in some decent place." No further elaboration is given, but I have heard of it being buried in the ground (surely more dignified than being flushed down the toilet).
More information on these and other Eucharistic mishaps can be found in the papal bull De Defectibus ("On Defects").