The Spinning Egg

More of a physics question than a puzzle this week, but I like it.

Take an empty plastic Easter Egg (a hard-boiled egg works, but a plastic one is easier and has less potential for egg destruction). Place it on its side on a flat surface and spin it quickly. After a few moments of spinning on its side, it will pop up and spin on one end. Why?

Source: Original.


Solution
Mail to Ken