Question: When is a Pope speaking ex-cathedra?

SacramentoTom asks (11/7/08): How do you know when a Pope in the past was infallible when he was speaking, and when he was just lounging around his home just talking regular. We know when Popes have spoken ex-cathedra since the 1800's when the doctrine of Infallibility of the Popes was introduced, but how do you know when they were speaking ex-cathedra (infallibly) prior to the 1800's? 

Answer: It is not hard to tell. Just like most people can tell the difference between: 

• when a priest is celebrating Mass and say the Last Supper words, "This is my body... This is my blood" 
• from when he is teaching a class describing the consecration at Mass where the priest says, "This is my body... This is my blood," 
• from when the priest is lounging around the house and says, the words, "This is my body... This is my blood." 

Likewise, most people can tell the difference when a couple says the words, "I do" at a wedding before a minister, and when a little 5 year old boy and girl say those words to each other at the wedding reception or when playing together outside or when lounging around the house. Again, they are the same words but have two distinct effects.

The same is true with the Pope. When he speaks ex-cathedra—from the chair of St. Peter—that is, when the Pope exercising his authority as Pope.