Here's why:
* Short vowels are single, quick sounds like the "a" in "cat" or the "i" in "big."
* Diphthongs are two vowel sounds blended together, like the "oi" in "boy" or the "ou" in "house."
The vowel sound in "higher" is actually a combination of two sounds:
1. The "i" sound as in "hit"
2. The "ə" (schwa) sound as in "about"
This combination creates a gliding sound that is characteristic of a diphthong.