The first article in the first Straight Dope book – the question was from a woman gazing comtemplatively at her stuffed lion, which had a belly button, and wondering if the King of the Jungle really did have a navel.
Well. Needless to say, I have worshipped Cecil as a demigod ever since.
Stompy
My first was the one about rectal foreign bodies in one of the books that a friend pointed out to me (in 1993, I believe), immediately followed by Can Cecil solve this geometry stumper?. It was the latter that got me hooked; any decent writer can make up stuff about folks sticking things up their butts and present it as fact, but you can’t fake a geometry proof. I stubled across the website sometime last summer, and the rest is history.
We really need to get Arnold in here to link all of these and tell us which book they’re in 
My first introduction was through alt.humor.best-of-usenet where someone posted a thread about farting etiquette in public restrooms from the alt.fan.cecil-adams newsgroup last year. I showed it to my sister Kiffa and she found the straightdope site. After the first couple of articles, I was hooked (spent several days reading them all).