Here's why:
* Short vowels have a short, clipped sound. Think of the "a" in "cat," the "e" in "bed," the "i" in "pig," the "o" in "hot," and the "u" in "cup."
* The "a" in "can" has a long vowel sound. It sounds like the "a" in "say" or "rain."
Therefore, "can" is considered a long vowel word.