simple Animation Windows Phone Draw Random Circles on screen

Question

I am now starting with the Windows Phone application based on Graphics for low level graphics operations.

I am using the Canvas as Background and one button called Start Button.

When I Click the Start button will clear the screen and start the animation thread and replace the start button with a stop button.

When I Click the Stop button will stop the animation thread and replace the Stop button with a Start button.
And the animation thread will draw a red circle object say Red Circles of same size at a random location within the width and height of the current screen.

How I can write code, can anyone can provide me code for this.