Here's why:
* The vowel sound in "teacher" is the sound of the letter "e" in the word "bed." This is a short vowel sound.
* Long vowel sounds are typically associated with a vowel followed by a silent "e" at the end of a word. For example, "ate," "bike," or "hope."
* In "teacher," the "e" at the end of the word is not silent. It is pronounced as part of the "er" sound.
Therefore, the vowel sound in "teacher" is a short vowel sound.