Here's why:
* Long vowels are typically associated with a vowel sound that is pronounced the same as the letter's name (e.g., "a" in "cake", "e" in "tree", "i" in "kite", "o" in "go", "u" in "cube").
* In "tooth", the vowel "oo" makes a short "oo" sound, similar to the "oo" in "book" or "look".
Therefore, "tooth" is a short vowel word.