Here's why:
* Short vowel sounds are typically quick and sharp. They are often found in words with one syllable and are typically followed by a consonant.
* Long vowel sounds are typically drawn out and often sound like the letter name. They can be found in words with multiple syllables or in one-syllable words with certain vowel combinations.
Since "soup" has a short, quick vowel sound similar to the "oo" in "look", it is considered a short vowel sound.