Sometimes you feel like drawing something, but you're not sure what.  Whenever you're in the mood to draw, you should just sit down and start drawing and let the pen or pencil or mouse go where it may.  Sometimes you get interesting results.

With this picture, when I decided to draw something I just started drawing.  After completing the first little squiggle I said, "Oh hey, it's a duck," and I continued the picture from there.  When you just start making squiggles and shapes, you can often draw inspiration from them and they develop into a nice little picture.