a) What is the Holy Eucharist?
The Holy Eucharist is the sacrament which contains the living Body, Blood, Soul, and Divinity of our Lord Jesus Christ, under the appearances of bread and wine. Christ instituted the Holy Eucharist at the Last Supper, the night before he died. “…Jesus took bread, said the blessing, broke it, and giving it to his disciples said, ‘Take and eat; this is my Body.’ Then he took a cup, gave thanks, and gave it to them, saying, ‘Drink from it, all of you, for this is my Blood of the covenant…” (Mt 26:26-28).
When our Lord said, “This is my Body,” the bread was changed into his Body; and when he said, “This is my Blood,” the wine was changed into his Blood (this conversion is called “transubstantiation”). Priests exercise this same power, given to them by Jesus, by repeating at the consecration of the Mass the words of Christ: “this is my Body… this is my Blood.” After the bread and wine have been changed into our Lord’s living Body and Blood, there remain only the appearances (size, color, shape, taste, weight, and texture) of bread and wine. He is present whole and entire in all the consecrated hosts and wine all over the world, and in each of the portions into which a consecrated host or wine is divided, since only the appearances of bread and wine are divided while the living Christ is not divided.
b) Why should an Altar Server thank our Lord for remaining with us always in the Blessed Sacrament?
The Holy Eucharist is the most excellent of all the sacraments. All the sacraments bring us grace, but only in the Eucharist is the source of all graces, Christ himself, truly present. All the other sacraments are directed towards the Holy Eucharist as their purpose.
We, along with our friends, can adore our Lord by visiting him in the Blessed Sacrament, going to Mass and receiving Holy Communion daily if possible, reverence in Church, genuflecting without rush (and perhaps saying interiorly, “My Lord and my God” or another aspiration), serving in the Mass and the Benediction of the Blessed Sacrament, greeting the Lord in the tabernacle upon arriving and before leaving the parish premises (or the school, if there is a chapel), and by saying the Spiritual Communion often during the day.