Here's why:
* Short vowel sounds are typically quick and sharp. In "knock," the "o" sound is pronounced like the "o" in "hot" or "lot."
* Long vowel sounds are typically drawn out and can be represented by a single vowel with a long mark over it (e.g., "ā," "ē," "ī," "ō," "ū").
* The vowel sound in "knock" is a short "o" sound, making it a short vowel.