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.
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.