The Holy Eucharist is the body of Our Lord just as long as the appearances of bread and wine remain, and when they go away Our Lord's body goes also. For example, if a church, tabernacle and all, was buried by a great earthquake, and after many years the people succeeded in getting at the tabernacle and opening it, and then found in the ciborium--that is, the vessel in which the Blessed Sacrament is kept in the tabernacle--only black dust, Our Lord would not be there, although He was there when the church was buried. He would not be there, because there was no longer the appearance of bread there: it had all been changed into ashes by time, and Our Lord left it when the change took place. But if the appearance of bread had remained unchanged, He would be there even after so many years. When we receive Holy Communion, the appearance of bread remains for about fifteen or twenty minutes after we receive, and then it changes or disappears. Therefore during these fifteen or twenty minutes that the appearance remains Our Lord Himself is really with us; and for that reason we should remain about twenty minutes after Mass on the day we receive, making a thanksgiving, speaking to Our Lord, and listening to Him speaking to our conscience. What disrespect some people show Our Lord by rushing out of the church immediately after Mass and Holy Communion, sometimes beginning to talk or look around before making any thanksgiving! When you receive Holy Communion, after returning to your seat you need not immediately begin to read your prayerbook, but may bow your head and speak to Our Lord while He is present with you. After the appearances of bread vanish, Our Lord's bodily presence goes also, but He remains with us by His grace as long as we do not fall into mortal sin. *246 Q. What is this change of the bread and wine into the body and blood of Our Lord called? A. This change of the bread and wine into the body and blood of Our Lord is called Transubstantiation. "Transubstantiation"--that is, the changing of one substance into another substance; for example, the changing of the wood in a seat into stone. *247 Q. How was the substance of the bread and wine changed into the substance of the body and blood of Christ? A. The substance of the bread and wine was changed into the substance of the body and blood of Christ by His almighty power. *248 Q. Does this change of bread and wine into the body and blood of Christ continue to be made in the Church? A. This change of bread and wine into the body and blood of Christ continues to be made in the Church by Jesus Christ through the ministry of His priests. 249 Q. When did Christ give His priests the power to change bread and wine into His body and blood? A. Christ gave His priests the power to change bread and wine into His body and blood when He said to His Apostles, "Do this in commemoration of Me."