Here's why:
* Short vowel sounds are typically quick and sharp. Think of the "o" sound in words like "hot," "lot," and "stop."
* Long vowel sounds are typically drawn out and have a more open sound. Think of the "o" sound in words like "go," "toe," and "boat."
The "o" in "dock" sounds similar to the "o" in "hot," making it a short vowel sound.