Here's why:
* Short vowel sounds are typically quick and sharp. Think of the sound you make when you say "a" in "cat," "e" in "bed," "i" in "big," "o" in "hot," or "u" in "cup."
* Long vowel sounds are typically drawn out and have a more open, relaxed sound. They are often pronounced the same way as the letter itself.
The "o" in "tock" makes a short, sharp sound, similar to the "o" in "hot."