There is an unwritten English rule concerning ablaut reduplication, or the altering of the interior vowels of a repeated word. When repeating words, the first vowel is always an I, then A or O if there are only two words, and I, A, O if there are three words. For example, it’s “chit-chat” and not “chat-chit”. – WTF Fun Facts
Vowels
WTF Fun Fact – Hawaiian Words
There are over 100 words in Hawaiian that are made up of only vowels. – WTF Fun Facts
Source: https://digitalcommons.butler.edu/cgi/viewcontent.cgi?article=1802&context=wordways