Singers work with sound the way painters work with color. It is not enough for a painter to think 'Blue...': Through their specialised knowledge of colour, the painter will envisage a very specific shade of blue, and know how to mix up the paints to get the desired result. So, in singing, it is not enough to simply think 'Ah' or 'Oo: the sound must be exactly the sound the singer desired to make. We must develop an inner 'ear' that hears and decides on the vowel shadings before they are produced.
Not any old vowel will do, though: some sounds are resonant, while others are not. It's a bit like the difference between a good flute and a cracked bamboo pipe. We will deal with vowels later, but right now we first need to understand a bit about what sound is and how it behaves.