This is one of the core tenants of Catholicism, the basic idea being that the Eucharist is not merely symbolizing the flesh of christ but actually becomes the flesh. Now lets get one thing out of the way first… gross (also does this mean that Catholics practice Dei-ivorism?).
Ok, on with the show. So, in this doctrine we have what may appear to be an empirical claim, the bread and wine turn into flesh and blood. This on the face of it would be something amenable to investigation that would show it to either be true or false. Unfortunately this tenant is more slippery than that, at least in the eyes of the church, for while the substance of the bread and wine do indeed become flesh and blood they still look, taste, smell and feel like, well, bread and wine.
So in what way can they be said to change if in all ways that can be measured they stay the same? Ah, that is the mystery. HUH? You see the substance of a thing is not defined by it’s appearance or constituent parts but is intrinsic to what it is. Thus a thing’s substance may change without altering any physical attribute, at least that’s the rationale given by the church. If you think that sounds like gobbledy gook, you’re not alone. Your average catholic can’t really wrap their head around it either. Thus the day is set to be saved by the antithesis of rational thought, the divine Mystery. Saying a particular piece of doctrine is a Mystery means it’s ok if it makes absolutely no logical sense and you don’t have to think about it.
Now I don’t want to appear to be insensitive but there does seem to be a clear gap between what logic and objectively verifiable evidence would tell us and what many of us are told to take on faith. I’ve written before that faith and reason are inherently mutually exclusive, if you have one the other is superfluous and even damaging. I know which one I’d rather put my trust in, do you?