Here's why:
* Short vowels are typically followed by a single consonant. In "fantastic," the vowel sounds are "a" (as in "cat"), "i" (as in "bit"), and "a" (as in "cat") again. Each of these vowels is followed by a consonant blend (st, nc, st).
* Long vowels can be followed by a consonant blend, a consonant digraph (two letters that make one sound), or be at the end of a word. In "fantastic," all three vowels are long.
Let me know if you have any other words you'd like to check!