Here's why:
* Long vowels are typically associated with a vowel that makes its own sound (like "a" in "cake" or "e" in "tree").
* Short vowels are typically associated with a vowel that makes a short, quick sound (like "a" in "cat" or "e" in "bed").
In the word "crab," the vowel "a" makes a short, quick sound. Therefore, it is a short vowel word.