with note

//setup() can set the screen size and bakground color , is called by Processing once should not be called again
//only be one setup()function for each sketch

void setup()//define a method or function
{
  size(500,400);
}

//you should never call the draw() function processing autocalls it after the setup() call
//draw()continuosly executes the cdoe ontained inside it, there can only be one draw()function for each sketch

void draw()
{

 //set to black,grayscale spectrum
 
 background(0); 
  noStroke(); //stroke(60,220,90) or stroke(0,0,0); //black outline
  fill(60, 220, 90); //green
  rect(0,100,width,40); //horizontal
  rect(100,0,30,height);//vertical
  ellipse(mouseX,mouseY,100,100);
  
}
 
 /*why many circles? background(0) is in the setup function*/
 /*what happens moves to background(0) into the draw function, before each cicle is drawn,the backgound is painted black, so it clears the previous circle*/