Here's why:
* Short vowel sounds are typically quick and sharp. The vowel sound in "rug" is a short, quick sound, similar to the "u" in "cup" or "sun."
* Long vowel sounds are typically drawn out and can often be spelled with a vowel followed by a silent "e" or a vowel pair. The vowel sound in "rug" does not fit this description.
Therefore, "rug" has a short vowel sound.