Here's why:
* Articles are words like "a", "an", and "the" that specify whether a noun is singular or plural and whether it's definite or indefinite.
* Vowel sounds include the sounds represented by the letters A, E, I, O, and U.
* The article "an" is used before words that start with a vowel sound, regardless of the actual letter they begin with.
Examples:
* an apple (the word "apple" starts with a vowel sound)
* an hour (the letter "h" is silent, so the word starts with a vowel sound)
* a university (the letter "u" is pronounced as a consonant sound)
Let me know if you have any more questions!