Here's why:
* Short vowel sounds are typically found in closed syllables (a syllable that ends with a consonant).
* "Supper" has one syllable, and it ends with the consonant "r".
* The sound of "e" in "supper" is a short, quick sound, like the "e" in "bed" or "ten".
Therefore, the "e" in "supper" is a short vowel sound.