Wikipedia has some learning for you. Abridged version–the Duomo is very glittery; when the sun shines on the front it is gorgeous. Story goes, that in Bolsena, Italy, a traveling priest doubted the transubstantiation as occurs in the Eucharist. His Host began bleeding, bled more, and stained the alter cloth. Upon seeing this miracle, the priest’s faith was restored. This alter cloth is now stored in the Duomo. During Corpus Christi celebrations, the Corporal of Bolsena is displayed on parade around the city.

Now after this Catechism lesson, here’s for the science believers! (Straight from Wikipedia, so you know it is true!)

It is only within the last ten years that the Academies of Science would not have been sorely puzzled to explain so strange a phenomenon. Now, no one thinks of denying it, since the discovery of a microscopic fungus, the spores of which having germinated in the meal or dough, offer the appearance of clotted blood.

Le Jardin de l’Epicure, Anatole France