Here's why:
* Diphthong: A diphthong is a single vowel sound produced by a continuous change in the position of the tongue and/or lips. It starts with one vowel sound and glides to another.
Examples of diphthongs in English include:
* "oi" in "boy"
* "ow" in "cow"
* "ay" in "day"
* "ee" in "see"
* "oo" in "moon"
Let me know if you'd like more information about diphthongs!