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var w = 10;
function setup(){
  createCanvas(400,400);
}

function draw(){

  for(var x = 0; x < width; x+=10){
    for(y = 0; y < height; y +=10){
      if((mouseX > x && mouseX < x + 10)&&(mouseY > y && mouseY < y+10)){
        fill(0,10);
        w= 20;
      }else{
        fill(255,10);
        w = 10;
      }
      rect(x,y,w,w);
    }
  }

}