Here's why:
* Short vowel sounds are typically quick and sharp. Think of the sound in words like "cat", "bed", "dog", and "cup".
* Long vowel sounds are usually drawn out and can be represented by a vowel followed by a silent 'e' at the end of the word. Think of the sound in words like "cake", "bike", "rope", and "tune".
The "u" in "cube" has a short, quick sound, similar to the "u" in "cup".