When a woman laughed at the Eucharist, thinking it was only bread, St. Gregory prayed and the host turned to flesh.
The Catholic Church teaches that after the consecration takes place at Mass, the Eucharistic host substantially changes into the body, blood, soul and divinity of Jesus Christ. This means that while the appearances of bread and wine remain, the substance is changed (through the power of God) completely to the body and blood of Christ.
It is a great mystery of the Church, but on occasion, God lifts the veil and allows even the appearances to change!
Such was the case in a story the Golden Legend told of St. Gregory the Great.
According to the story, St. Gregory was about to give communion to a woman who baked the bread used at Mass. The woman surprisingly started to laugh, because she thought it was ridiculous to think that the bread she made was the body of Jesus.
It happed that a widow brought hostsevery Sunday [for the priest] to [celebrate] Mass with … when Saint Gregory [was about to] give to her the holy sacrament in saying, [he said] May the body of our Lord Jesus Christ keep you into everlasting life … this woman began to laugh at Saint Gregory, and he withdrew his hand, and placed the sacrament upon the altar. And he asked her, before the people, why she laughed, and she said: Because that the bread that I have made with my proper hands, you call the body of our Lord Jesus Christ.
Saint Gregory put himself to prayer with the people, for to pray to God that hereupon he would show his grace for to confirm our belief, and when they were risen from prayer, Saint Gregory saw the holy sacrament in figure of a piece of flesh as great as the little finger of an hand, and by the prayers of Saint Gregory, the flesh of the sacrament turned into appearance of bread as it had been before, and therewith he gave communion to the woman, which after was more religious, and the people more firm in the faith.
This story is similar to many other Eucharistic miracles, where the faith of a person, or even a priest, was failing, and they were given a sign of the Real Presence of Jesus in the host.
God allows miracle such as this to enliven the faith of all and to reinforce the reality that he truly is present in the host at Mass, body, blood, soul and divnity.