Here's why:
* Short vowels are usually followed by a single consonant. In "cereal", the vowel "e" is followed by two consonants ("r" and "a").
* Long vowels often have a vowel digraph (two vowels together) or a silent "e" at the end of the word. In "cereal", the "ea" makes the "e" sound long.
Therefore, "cereal" has a long vowel sound.