Android :: Draw 9 Patch Tool - What Is Meaning Of Black Lines Outside Figure

I tried enough to find in google & android this question. Also did several trials & errors, but couldn't get this question. was learning Android's "Draw Nine Patch" images. One basic I know is that, whatever image you process must be in .png format (if it's already in .9.png then it will be ignored by tool). However, couldn't understand what it literally means by "stretchable patches" ? what exactly happens when you draw black dots outside the .png image ? (preview pane shows changes happening, but I am unclear on how it makes those)When I draw black dots in left/top sides of image it shows some changes in preview; but why isn't there any effect when you do the same on right/bottom side of the image ? why this tool is used primarily for background, when we try to process on the image itself ?i am trying to modify a simple button.png (given in android sdk). Whenever I use this tool & draw black lines, it reduces the size of the image instead of 'stretching' it! Why ?What is the significance of options given below like, "show lock", "show content", "Patch scale" and so on ?I apologize for asking so many questions, but in Android online docs they haven't explained well for novice. If someone can answer these, it will become ready reference for all the begineers who search this forum.