system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: As a kid I was told the vowels were a, e, i, o, u and sometimes y, why does y only count sometimes?
gpt
Letters aren't inherently vowels. Sounds are. 

As it happens, the sounds that a, e, i, o, and u generally make are all vowels sounds. The letter y is kind of weird in it can make a yuh sound (as in yam), or an eee sound (as airy). In the former case it is not serving as a vowel, while in the latter it is.