Here's why:
* Pronunciation: The choice of "a" or "an" depends on the sound the word starts with, not the letter itself. If the word sounds like it starts with a vowel, use "an".
* Examples:
* An apple (The "a" in "apple" sounds like a vowel)
* A university (The "u" in "university" sounds like the consonant "y")
* An hour (The "h" in "hour" is silent, so the word sounds like it starts with a vowel)
Remember, it's all about the sound, not the letter!