Here's why:
* The vowel sound in "grass" is the same as the vowel sound in "car". This sound is represented by the letter "a" followed by the letter "r".
* Short vowels are typically represented by a single vowel letter in a word. For example, "cat" has a short "a" sound.
Therefore, "grass" has a long "a" sound, which is represented by the vowel digraph "a-r".