Here's why:
* The vowel sound in "crack" is similar to the vowel sound in "cat", "hat", or "bag". These words all have short vowel sounds.
* The vowel sound is short and sharp, not drawn out.
* The "a" in "crack" is followed by a consonant cluster ("ck") which typically makes the vowel sound short.
Let me know if you have any other words you'd like to check!