Here's why:
* Short vowel sounds are typically quick and sharp. In "sat," the "a" sound is short and clipped.
* Long vowel sounds are typically drawn out. The "a" in "sat" is not drawn out.
To illustrate:
* Short vowel sound: "sat" sounds like the "a" in "cat" or "hat".
* Long vowel sound: "say" sounds like the "a" in "cake" or "hate".