A diphthong is a vowel sound where your mouth changes position during the sound, starting with one vowel and gliding towards another.
Here are some examples of English diphthongs:
* "oi" as in "boy"
* "ou" as in "house"
* "ai" as in "rain"
* "au" as in "cow"
* "ey" as in "say"
Let me know if you'd like more examples or want to learn about other vowel sounds!