// Click on the image to give it focus
// and press any key

// Note: the rectangle in this example may 
// flicker as the operating system may 
// register a long key press as a repetition
// of key presses

void draw() {
  if (keyPressed == true) {
    fill(0);
  } else {
    fill(255);
  }
  rect(25, 25, 50, 50);
}