Want to end a conversation about the Real Presence before it hardly begins? Utter just one word: Transubstantiation. Eyes glaze over, minds go blank, and your listeners head for the door. If you want to talk about the Real Presence, particularly with those who never have heard of it, you need to begin with a concrete object. Let’s use your shirt.
Its color is not the shirt itself, nor is its shape, its soft feel, or its (freshly laundered) scent. No one thing that your senses can perceive about the shirt is the shirt itself, and no combination of such things is the shirt itself. Your senses, on their own, are not capable of knowing fully what your shirt is. They miss its underlying nature. But your mind perceives the shirt’s underlying nature because it is able to bring together the information your senses gather about the shirt, and it extrapolates from there. It perceives the "shirtness" of your shirt, and "shirtness" is more than just the shirt’s color, shape, feel, or scent. In centuries gone by, philosophers called the underlying nature of a thing its "substance" and called its outward, sensible manifestations its "accidents." Since that last word conjures up images of colliding automobiles, let’s use "appearances" instead.
It is easy enough to imagine what happens if the appearances of a thing are changed. If the dye applied to your shirt is changed from white to red, the shirt’s color changes, and you would see the difference. If the sleeves are lengthened or shortened, the shirt’s shape changes. If the material is changed from polyester to cotton, the shirt’s feel changes. If you wear the shirt while running a mile and then do not wash it, its scent changes. It remains a shirt no matter which of these changes is made. But what if the substance of the shirt is changed while the appearances remain constant? What if its "shirtness" is replaced by "shoeness" or "hatness" while its color, shape, feel, and scent remain the same? Your mind would be unable to detect the change.
This is what happens in the Eucharist. The substance of the bread is changed into Christ’s body, while the appearances of the bread are unaltered. The color, shape, feel, and scent of the bread all remain. Working only on data provided by your senses, your mind would say no change has occurred. You could use the most delicate scientific instruments but would find no alteration of the bread’s outward manifestations. This is not surprising, since science is incapable of dealing with a thing’s substance. It is limited to what the senses can perceive and thus to a thing’s appearances. It cannot detect whether substance has changed because substance is imperceptible.
So how do we know that, at the moment of the consecration, the bread ceases to be bread in its substance and becomes instead the body of Christ—that its substance ceases to be one thing and becomes another? We know it because God tells us so, through his Church, which got the information from Christ himself (see the sixth chapter of John’s Gospel).
At the consecration we have a change in substance. Something that had one substance (bread) now has another (the body of Christ), even while the thing’s outward appearances remain static. This change in substance, this "going over" (we can use the Latin prefix "trans") from one substance to another is called "transubstantiation." This is a good word for technical manuals and for discussions among the well-informed. It is a theological word of art that can seem artless or even off-putting to those not familiar with the idea of "substance." It is worth using, but only if the proper groundwork has been laid.