Here's why:
* Short vowel sounds are typically quick and sharp. They are often found in words with one syllable and one vowel.
* Long vowel sounds are typically drawn out and can sometimes sound like the vowel's name.
The "a" in "jam" has a short, quick sound, similar to the "a" in "cat" or "man". It's not the drawn-out sound of the long "a" in words like "cake" or "name".