Here's why:
* The schwa sound (ə) is the most common vowel sound in English. It's a short, unstressed vowel sound, often found in unstressed syllables.
* In "rustic", the second syllable "tic" is unstressed. The vowel sound in "tic" is the schwa sound.
Therefore, the word "rustic" is pronounced with a schwa sound in the second syllable.