What is transubstantiation?

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 http://en.wikipedia.org/wiki/Transubstantiation 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*?