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According to Wikipedia:

When at his Last Supper, Jesus said: "This is my body", what he held in his hands still had all the appearances of bread: these "accidents" remained unchanged. However, the Roman Catholic Church believes that, when Jesus made that declaration, the underlying reality (the "substance") of the bread was converted to that of his body. In other words, it actually was his body, while all the appearances open to the senses or to scientific investigation were still those of bread, exactly as before.

and also

The Catholic Church holds that the same change of the substance of the bread and of the wine occurs at the consecration of the Eucharist

So, how can the Eucharist change from bread to body if it is not measurably altered in any way? How is it different from saying that it doesn't change at all? Is it an allegory? If not, how does it change in practical terms?

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related:… – Richard Sep 12 '11 at 20:41
Side note: there are documented cases where the accidents of bread and wine were changed too. – Kristo Sep 13 '11 at 2:43
Check out Dr. Anthony Rizzi's explanation of transubstantiation to his 7 year old daughter in the DVD Physics for First Communion. – Geremia Aug 23 '15 at 1:04

Transubstantiation relies on the Aristotelian distinction between essence and accidents. From the linked Wikipedia article:

Catholic theologians such as Thomas Aquinas have employed the Aristotelian concepts of substance and accident in articulating the theology of the Eucharist, particularly the transubstantiation of bread and wine into body and blood. According to this tradition, the accidents of the bread and wine do not change, but their substances change from bread and wine to the Body and Blood of Christ.

The "measurable" properties you are referring to (taste, feel, etc.) are accidental properties, not the substance of the bread/wine.

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So in practice "it's actually the Body, but 'masquerades' as bread"? – Sklivvz Sep 12 '11 at 20:43
Catholics probably would not be happy with the term 'masquerades'. But yes, if what you mean is that after consecration the Body retains all of the outward and visible properties of the bread. – gmoothart Sep 12 '11 at 21:00
@Sklivvz As predicted by gmoothart, I (a Catholic) don't particularly care for the term 'masquerade' (it just feels irreverent), but that is probably one of the best single word terms that can describe the reality of transubstantiation. – cledoux Sep 13 '11 at 1:23

Practically, it changes in the sense that you now believe that the Eucharist is God made present before you.

Previous to consecration it required no faith to believe that bread and wine were bread and wine. After consecration it requires absolute faith to see Our Lord before us.

Transubstantiation is simply a miracle and like most miracles your freewill and liberty are sublimely preserved even when it happens in front of your eyes.

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I don't believe that, in Catholic doctrine, faith is required for the bread to be be transubstantiated into the Body. Faith is required for it to be effective, but it (at least the substance as gmoothart says) is actually transformed. – DJClayworth Sep 12 '11 at 20:54
@djc No, you're right, nothing is required except that the priest had said a valid Mass and words of consecration. The way this question was worded requires some careful explanation to actually mean anything. – Peter Turner Sep 12 '11 at 21:05
@PeterTurner God made present before you More accurately Jesus Christ whole: His body, Blood, Soul, and Divinity. – user13992 Jul 24 '14 at 18:23

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