The Continuing Mystery of "Self-Combustible" Wool

I was listening to Car Talk (in podcast form) on the bus ride home this afternoon, when I heard a new reference to wool being self-combustible! In answering last week's puzzler, Ray was reading from Philip McCutcheon's book "Tall Ships":

Spaced along the upper deck were the cargo hatches with their heavy covers of reinforced hardwood planks, well chocked in and secured with three separate layers of tarpaulin, held down with ropes and more chocks to withstand the pounding of heavy seas. Below the hatches lay the reason for the ship's presence on the sea, her cargo, to be held inviolate against nature and disaster, against fire that could come from a self-combustible cargo like wool, or a cargo that could swell when it met water, such as rice.

So, seeing a possible opportunity to get an answer to this burning question, I sent Car Talk the following e-mail:

I have two questions for you, one which has been bothering me for a long time, and one that came up today as I listened to the podcast of the latest show.

My first question relates to terminology used on the show. One of the brothers always announces that the new puzzler will be given in the "third half" of the show. Mathematically, unless each show is actually a show and a half, there can only be two halves to each whole show. Is the "third half" terminology something standard in radio and or showbusiness (and if so, why?), or are Click and Clack just doing their "sounding stupid" thing?

My other question is actually something that's been bothering me for a long time as well, but only thought of writing Car Talk after hearing the Puzzler answer. In the answer, Ray reads from Tall Ships, which refers to "a self-combustible cargo like wool." Some of my knitter friends have not been able to mail packages after saying the contents are wool, because the post office also has the notion that wool is self-combustable when wet. Now, as a knitter, I have never had my wool self-combust, not even when washing it. I have never heard of sheep in Britain (where it is certainly very damp much of the year) self-combusting either. In fact, one test of whether or not yarn is indeed wool is to hold it to a lit match, and then see if it *self-extinguishes* when the match is taken away. I can only imagine the wool would be even less combustible when wet... unless it was wet with gasoline. (At which point, I think just about anything would be considered self-combustible.) Could you please help me and my knitting friends understand this?

Thank you!

I'll let you know if I get an answer. ;)