Here's why:
* Schwa sound: The schwa sound is a very short, unstressed vowel sound that is represented by the upside-down "e" symbol (ə) in the International Phonetic Alphabet (IPA). It's the most common vowel sound in English and is often found in unstressed syllables.
* "Stomach": The first syllable ("sto") is unstressed, while the second syllable ("mach") is stressed. The "o" in the first syllable is a very short and indistinct sound, making it a schwa.
So, the schwa sound in "stomach" is the "o" in "sto."