void setup(){
  size(400,400);
  fill(255,0,0);

  for( int i = 0; i<400; i+=20){
    line (i,0,i,height); //vertical height
  }

  for (int i =0; i<400; i+=20){
    line(0,i,width,i); //hortizontal lines
  }


}

void draw(){
}

void mousePressed(){
rect(mouseX - mouseX%20 , mouseY - mouseY%20, 20, 20);
//click the mouse and it generates a rectangle at a point on the grid near where you clicked.
//mouseX has the remainder of mouseX divided by 20 subtracted, so that it lands on a multiple of 20. same for mouse Y
//since the grade is split into multiples of 20, this ensures that a square is drawn on the grid inside of the pre-exisitng lines
//this makes the grid filled with red


}