Here's why:
* Short vowel sound: The "a" in "baby" has a short vowel sound, similar to the "a" in "cat" or "hat". It is pronounced as /æ/.
* Long vowel sound: A long vowel sound would be pronounced as the vowel's name, like "a" in "cake" or "e" in "see".
Therefore, "baby" has a short vowel sound in the "a".