I must be missing something really stupid.  Why doesn't the following code continually fill the window with white? 

(I know this is inefficient...I'm just boiling down a more complex idea (that requires rects) to the most basic)

void setup(){
  size(50,50);
}

void draw(){
  fill(255,255);
  for(int y=0;y<height;y++) {
    for(int x=0;x<width;x++) {
        rect(x,y,1,1);
    }
  }
}