void setup() {
  size(400,400);
}

void draw() {
  background(220);
  
  stroke(80);
  strokeWeight(30);
  smooth();
  line(mouseX,mouseY,pmouseX,pmouseY);
  
  noFill();
  stroke(180);
  strokeWeight(10);
  rect(10,10,width-20,height-20);
  
}