I cut and paste this post from LauraL on another thread under Family Life. I think it’s an excellent explanation. I hope it helps you as much as it did me.
She said:
I recommend you tell him that the Church, thanks to St.Thomas Aquinas and a Greek philosopher named Aristotle (whose work in philosophy helped St. Thomas in his theological studies) uses a one-hundred dollar word called TRANSUBSTANTIATION to explain what happens at the Consecration.
Aristotle figured out that everything that is, has two parts of its being – its Substance and its Accident. The Substance is what makes something that thing. For example, a tree is not a flower nor a shrub nor a beanstalk. We say “tree” and we automatically think of all those things that distinguish “tree” from everything else that exists. That’s the Substance. But you could speak of a pine, a maple, or a dogwood, and all those particulars that distinguish one tree from another are the Accidents.
You could even use “Man” as an example. All those things that we think of when we say Man are the Substance – when we think of Man and not woman or boy or girl or cocker spaniel. But the Accidents are those particulars that distinguish you from Grampa or from Uncle Bob or Mr. Smith down the street.
Now – you see the word? TranSUBSTANTiation. The “Is-ness” of the bread and the wine are changed, their SUBSTANCE become literally the Body and Blood of Christ. Their Accidents remain the same, they don’t become muscle tissues or corpuscles (which is also why we’re not cannibals!). That’s why if you looked at consecrated Host or the Precious Blood under the microscope, you wouldn’t know that it was the Body of Christ, because the accidents, those physical particulars, are the same; it’s the Substance that God alters to be Himself.