Sing a long note and change its sound through all the vowels and the difference between the vowels is all in the relative strength of the overtones.

Your vocal chords actually produce a rich set of overtones (harmonics) all the time, but by changing the shape of your mouth you make it resonate with different overtones which emphasises some relative to the others.

Anahata
(I hope that made sense, it's before 9am and I haven't had my first coffee yet)