A Cow, An alligator?

Ok, this may seem really juvenile, but it wasn't until my senior year in high school that I finally realized why the computer liked to put green squiggles under my a's. And it wasn't until this year that I started to regulalry put an's in my papers (without spell check) and I still have to watch myself. So, I thought I would go over the rules for a and an.
A is used in front of consonants
A Jedi kicked a Sith.
An is used in front of vowels.
A Jedi kicked an Ewok. (and frankly, they deserve it)

http://owl.english.purdue.edu/owl/resource/540/01/