Here's why:
* Schwa sound: The schwa sound is a very short, unstressed vowel sound represented by the symbol ə in the International Phonetic Alphabet. It sounds like the "uh" in "about" or the "a" in "sofa".
* "Mys" syllable: The first syllable of "mystery" ("mys") is unstressed, meaning it receives less emphasis than the second syllable ("te-ry").
* Sound of the "y": In this case, the "y" in "mys" makes the schwa sound.
So, the word "mystery" is pronounced with a schwa sound in the first syllable: ˈmɪstəri.