Here's why:
* Short vowel sounds are typically quick and sharp, often represented by a single vowel letter in a word. In "nap," the "a" makes a short, quick sound.
* Long vowel sounds are typically drawn out and often represented by a vowel followed by a silent "e" or a vowel digraph (like "ee" or "ai").
Therefore, the "a" in "nap" makes a short vowel sound.