system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes a letter a vowel? Is that consistent across all languages?
gpt
A vowel is essentially a letter sound that is generated without constricting the airflow in your throat -- compared to a consonant which involves manipulating or constricting the air in some way.
Think of the three sounds that go into the word "Cat".  You have the C/K sound + the "a" sound + plus the hard T sound.  

Say the word slowly -- notice how in the K sound, you have to push a short burst of air over your tongue to the top of your mouth -- in the T sound you do something similar, pushing the air between your tongue and your teeth.

The "a" sound on the other hand, has a nice open mouth and unrestricted airflow in your throat.  That's a vowel.  Notice how much easier and natural it is to hold that "a" sound for a long time compared to the K/T.

Vowels are consistent in that basically all languages have them, because they give your throat and airflow a break from the "work" required in a consonant.  But how the vowels sound, how many there are, and how frequently they are used do vary wildly -- English has just 6 vowel letters, but those letters can make around a dozen different vowel sounds (think of the a in "apple" vs "cape").  Spanish, on the other hand, has just 5 vowel letters and each letter makes a more-or-less consistent sound.