It is easiest to tell what transubstantiation is by saying this: little children should be taught about it as early as possible. Not of course using the word...because it is not a little child's word. But the thing can be taught... by whispering. ."Look! Look what the priest is doing...He's saying Jesus' words that change the bread into Jesus' body. Now he's lifting it up. Look! Now bow your head and say 'My Lord and my God...' "
I have spoken of teaching little children, both because it is important in itself and because it is the clearest way of bringing out what "transubstantiation" means. That word was devised (first in Greek and then in Latin by translation) to insist precisely on this: that there is a change of what is there, totally into something else. A conversion of one physical reality into another which already exists. So it is not a coming to be of a new substance out of the stuff of an old one...Nor is it like digestion in which what you eat turns into you. For these are both changes of matter, which can assume a variety of forms. When one says "transubstantiation" one is saying exactly what one teaches the child, in teaching it that Christ's words, by the divine power given to the priest who uses them in his place, have changed the bread so that it isn't there anymore (nor the stuff of which it was made) but instead there is the body of Christ. The little child can grasp this and it is implicit in the act of worship that follows the teaching. I knew a child, close upon three years old and only then beginning to talk, but taught as I have described, who was in the free space at the back of the church when the mother went to communion. "Is he in you?" the child asked when the mother came back. "Yes," she said, and to her amazement the child prostrated itself before her. I can testify to this, for I saw it happen. I once told the story to one of those theologians who unhappily (as it seems) strive to alter and water down our faith, and he deplored it: he wished to say, and hoped the Vatican Council would say, something that would show the child's ideas to be wrong. I guessed then that the poor wretch was losing the faith, and indeed so, sadly, did it turn out.
From "On Transubstantiation" by Elizabeth Anscombe
Collected Philosophical Papers, V.III: Ethics, Religion, and Politics, 1981, Univ. of Minnesota Press